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Atoms, molecules and stoichiometry

csonn t1 atoms, molecules and stoichiometry

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Page 1: csonn t1 atoms, molecules and stoichiometry

Atoms, molecules and stoichiometry

Page 2: csonn t1 atoms, molecules and stoichiometry

Atoms and elements• An atom is the smallest particle into which an element can be

divided without losing its identity (the smallest indivisible particle of an element).

• A molecule is a group of atoms (held together by covalent bonding) which is capable of independent existence.

• Elements are pure substances that contain only one type of atoms.

• Ions are charged species formed as a result of loss of electrons (cations) or gain of electrons (anions) thus having an overall electrical charge.

• Isotopes are atoms of the same element that have the same number of protons but different number of neutrons.

Page 3: csonn t1 atoms, molecules and stoichiometry

Atomic structure

mass of one proton = ?

mass of one neutron = ?

mass of one electron = ?

Atomic StructureDetails of the three Sub-atomic (fundamental) Particles

Particle Position Relative Mass Relative ChargeProton Nucleus 1 +1

Neutron Nucleus 1 0Electron Orbit 1/1800 -1

73

LiAtomic Number

Atomic SymbolMass Number

The atomic number, Z, is the number of protons in the nucleus.

The mass number ,A, is the total number of protons and neutrons in the atom.

Number of neutrons = A - Z

Isotopes Isotopes are atoms with the same number of protons, but different numbers of neutrons.

Isotopes have similar chemical properties because they have the same electronic

structure. They may have slightly varying physical properties because they have

different masses.

THE MASS SPECTROMETER

Electrons

ionise the

Electric field

accelerates the

ions

The mass spectrometer can be

used to determine all the

isotopes present in a sample of

There are various

models for atomic

structure

Vaporised

sample

detector

Vacuum pump

ionise the

sample

ions

magnetic field

defects the ions

isotopes present in a sample of

an element and to therefore

identify elements.

The following are the essential 4 steps

in a mass spectrometer.

1.Ionisation •A Vaporised sample is injected at low pressure

•An electron gun fires high energy electrons at the sample

•This Knocks out an (outer) electron

•Forming positive ions with different charges E.g. Ti ! Ti+ + e–

2. Acceleration•A negative electric field accelerates the positive ions and makes them into a beam

3. Deflection •The beam of positive ions is deflected by a strong magnetic field.

•The degree of deflection depends on the mass-to-charge ratio, m/z.

•The smaller this ratio the larger the deflection.

•By varying the magnetic field ratio, ions of different m/z ratios pass through the centre.

4. Detection•The ions reach the detector and generate a small current, which is fed to a computer for analysis

Learn all these steps

carefully!

If the sample is not vaporized

then vaporizing it would be

the first step.

It needs to be under a vacuum

otherwise air particles would

ionise and register on the

detector

The heavier an ion the

less it is deflected

1

Copyright N Goalby Bancroft's School

Page 4: csonn t1 atoms, molecules and stoichiometry

Atomic structureParticle Position Actual mass Relative mass Actual charge Relative charge

Proton Nucleus ? 1 ? +1

Neutron Nucleus ? 1 ? 0

Electron Orbit ? 1/1800 ? -1

Page 5: csonn t1 atoms, molecules and stoichiometry

Atomic structure - deflection - A’Level physics

• The force on a charge q moving with a velocity v through a magnetic field B, is

F = Bqv

• Inside mass spectrometer, F = Bqv

• Where deflection occurs, the motion is assumed to be in circular motion.

• The centripetal force acting on an object moving in a circular motion is,

F = mv2

/r

• Hence, equating the two,

F = Bqv = mv2

/r

• r = radius of the ion path deflected = mv/Bq

• Electrons and protons have the same charge numerically but deflected in opposite directions.

• Protons is so much heavier than electrons, then r is smaller to maintain constant F - it get deflected less.

Page 6: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q1

11

! UCLES 2004 9701/01/M/J/04 [Turn over

Section B

For each of the questions in this section, one or more of the three numbered statements 1 to 3 may

be correct.

Decide whether each of the statements is or is not correct (you may find it helpful to put a tick against

the statements that you consider to be correct).

The responses A to D should be selected on the basis of

A B C D

1, 2 and 3

are

correct

1 and 2

only are

correct

2 and 3

only are

correct

1 only

is

correct

No other combination of statements is used as a correct response.

31 The isotope cobalt-60 ( Co60

27) is used to destroy cancer cells in the human body.

Which statements about an atom of cobalt-60 are correct?

1 It contains 33 neutrons.

2 Its nucleus has a relative charge of 27+.

3 It has a different number of neutrons from the atoms of other isotopes of cobalt.

32 The conversion of graphite has only a small positive value of !H.

C (graphite) " C (diamond) !H = +2.1 kJ mol –1

However, the production of synthetic diamonds using this reaction is very difficult.

Which statements help to explain this?

1 The activation energy of the reaction is large.

2 An equilibrium exists between diamond and graphite.

3 Only exothermic reactions can be made to occur readily.

33 Which statements about the properties of a catalyst are correct?

1 A catalyst increases the average kinetic energy of the reacting particles.

2 A catalyst increases the rate of the reverse reaction.

3 A catalyst has no effect on the enthalpy change !H o of the reaction.

Page 7: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q1

Page 8: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q2

2

© UCLES 2005 9701/01/M/J/05

Section A

For each question there are four possible answers, A, B, C, and D. Choose the one you consider to

be correct.

1 A pure hydrocarbon is used in bottled gas for cooking and heating.

When 10 cm3

of the hydrocarbon is burned in 70 cm3

of oxygen (an excess), the final gaseous

mixture contains 30 cm3

of carbon dioxide and 20 cm3

of unreacted oxygen. All gaseous volumes

were measured under identical conditions.

What is the formula of the hydrocarbon?

A C2H6 B C3H6 C C3H8 D C4H10

2 On collision, airbags in cars inflate rapidly due to the production of nitrogen.

The nitrogen is formed according to the following equations.

2NaN3 → 2Na + 3N2

10Na + 2KNO3 → K2O + 5Na2O + N2

How many moles of nitrogen gas are produced from 1 mol of sodium azide, NaN3?

A 1.5 B 1.6 C 3.2 D 4.0

3 The first six ionisation energies of four elements, A to D, are given.

Which element is most likely to be in Group IV of the Periodic Table?

ionisation

energy / kJ mol−1

1st 2nd 3rd 4th 5th 6th

A 494 4560 6940 9540 13400 16600

B 736 1450 7740 10500 13600 18000

C 1090 2350 4610 6220 37800 47000

D 1400 2860 4590 7480 9400 53200

4 In which species are the numbers of electrons and neutrons equal?

A Be9

4 B F

19

9 C

23

11Na

+

D 18

8O

2–

Page 9: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q2

Page 10: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q3

12

© UCLES 2007 9701/01/M/J/07

Section B

For each of the questions in this section, one or more of the three numbered statements 1 to 3 may

be correct.

Decide whether each of the statements is or is not correct (you may find it helpful to put a tick against

the statements that you consider to be correct).

The responses A to D should be selected on the basis of

A B C D

1, 2 and 3

are

correct

1 and 2

only are

correct

2 and 3

only are

correct

1 only

is

correct

No other combination of statements is used as a correct response.

31 Use of the Data Booklet is relevant to this question.

The technetium–99 isotope (99

Tc) is radioactive and has been found in lobsters and seaweed

adjacent to nuclear fuel reprocessing plants.

Which statements are correct about an atom of 99

Tc?

1 It has 13 more neutrons than protons.

2 It has 43 protons.

3 It has 99 nucleons.

32 Which of the following solids contain more than one type of chemical bond?

1 brass (an alloy of copper and zinc)

2 graphite

3 ice

33 Many crude oils contain sulphur as H2S. During refining, by the Claus process, the H2S is

converted into solid sulphur, which is then removed.

reaction I 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)

reaction II 2H2S(g) + SO2(g) → 2H2O(l) + 3S(s)

Which statements about the Claus process are correct?

1 H2S is oxidised in reaction I.

2 SO2 oxidises H2S in reaction II.

3 SO2 behaves as a catalyst.

Page 11: csonn t1 atoms, molecules and stoichiometry

Atomic structure - Q3

Page 12: csonn t1 atoms, molecules and stoichiometry

Relative masses• Carbon-12 (12C) isotope adopted by IUPAC as

reference standard for relative atomic masses.

• On the carbon-12 scale, an atom of the isotope 12C is assigned a mass value of 12 atomic mass units (a.m.u.)

• Overall, it is a dimensionless quantity (no physical units).

Page 13: csonn t1 atoms, molecules and stoichiometry

Relative masses - Important!

1.1 Atoms,Molecules & Stoichiometry "

All substances are made up of atoms. Understanding atoms can help us comprehendintriguing phenomena such as bonding and stmctt~e, and predict properties ofmatter.

Atoms An atom is the smallest particle into which an element can be divided without losingits identity; i.e. the smallest indivisible particle of an element.

Molecules A molecule is a group of atoms (held together by covalent bonding) which is capableof independent existence, e.g. hydrogen H2, water H20, ammonia NH3.

Isotopes Isotopes are atoms of the same element that have the same number of protons butdifferent number of neutrons; i.e. same atomic number but different mass numbers.

Most natural samples of an element are mixtures of two or more isotopes, e.g.

lsotope

35C11 7

3 7 C I1 7

mass no.

1 6

17

18

atomic no.(z)8

8

8

no. of neutrons

[See Atomic Structure in Section 1.2 ]

Atoms have very small masses (~ 10-23 g). Hence, a relative atomic mass scale (viz.~2C scale) is used to compare the masses of different atoms; i.e.

mass of one particlerelative mass of a particle =

1~ x mass of one atom of 12C isotope

The 12C scale is one on which an atom of ~2C isotope is assigned a mass ofexactly 12.000 units.

Carbon is chosen as the standard because it is a very common element which iseasy to store and transport (carbon being solid at room temperature).

RelativeAtomicMass

The relative atomic mass, Ar, of an element is defined as the ratio of the average

the mass of an atom of ~2C isotope,mass of one atom of the element to

expressed on the ~2C scale.

mass of one atom of an element± x mass of one atom of 12 C isotope1 2

O St~p-by-Step

Advanced Guide - Chemistry

Page 14: csonn t1 atoms, molecules and stoichiometry

Relative masses - Important!

1.1 Atoms, Molecules & Stoichiometry

The relative isotopic mass, Ar, of a particular isotope is defined as the ratio of thei the mass of an atom of 12C isotope,mass of one atom of the isotope to ]2

expressed on the 12C scale.

mass of one atom of a certain isotopex mass of one atom of 12C isotope

1 2

RelativeIsotopic

Mass

The relative molecular mass, Mr, of a substance is defined as the ratio of the average

mass of one molecule of the substance to ~ the mass of an atom of

12C isotope, expressed on the 12C scale.

Mr = mass of one moleculeof the substance± x mass of one atom of ~2C isotope1 2

Mr can be simply taken to be the sum of the Ar of all the atoms shown in the formulaof the substance.

e.g. Mr ofCaCO3 = 40.1 + 12.0+ 3(16.0) = 100.1

[NB. Relative atomic masses, Ar, are given in the Periodic Table.]

The relative formula mass of an ionic compound, Mr, is defined as the ratio of thethe mass of an atom ofaverage mass of one formula unit of the compound to

12C isotope, expressed on the lZc scale.

Mr = mass of one formula unit of the compound_x x mass of. one atom of 12 C isotope12

~" The term relative formula mass is used for all ionic and giant molecularcompounds, which do not form simple molecular entities.

~" Mr can be simply taken to be the sum of the Ar of all the atoms shown in theformula unit of the compound; the charge on the ion is ignored.

e.g. Mr of CO32-= 12.0+3(16.0) = 60.0

[NB. A formula unit refers to the smallest group of atoms from which the formulaof an ionic (or giant molecular) compound is determined.]

All the relative masses, both Arand Mr, have no units since they are ratios.

The molar mass of a substance is the mass of one mole of that substance.

Numerically, molar mass = Ar (or Mr) but has units of g mo1-1.

e.g. molar mass of CaCO3 - [40.1 + 12.0 + 3(16.0)] g mo1-1= 100.1 g mo1-1

© Step-by-Step

RelativeMolecular

Mass

RelativeFormula

Mass

Advanced Guide - Chemistry

Page 15: csonn t1 atoms, molecules and stoichiometry

Relative masses - Important!

1.1 Atoms, Molecules & Stoichiometry

The relative isotopic mass, Ar, of a particular isotope is defined as the ratio of thei the mass of an atom of 12C isotope,mass of one atom of the isotope to ]2

expressed on the 12C scale.

mass of one atom of a certain isotopex mass of one atom of 12C isotope

1 2

RelativeIsotopic

Mass

The relative molecular mass, Mr, of a substance is defined as the ratio of the average

mass of one molecule of the substance to ~ the mass of an atom of

12C isotope, expressed on the 12C scale.

Mr = mass of one moleculeof the substance± x mass of one atom of ~2C isotope1 2

Mr can be simply taken to be the sum of the Ar of all the atoms shown in the formulaof the substance.

e.g. Mr ofCaCO3 = 40.1 + 12.0+ 3(16.0) = 100.1

[NB. Relative atomic masses, Ar, are given in the Periodic Table.]

The relative formula mass of an ionic compound, Mr, is defined as the ratio of thethe mass of an atom ofaverage mass of one formula unit of the compound to

12C isotope, expressed on the lZc scale.

Mr = mass of one formula unit of the compound_x x mass of. one atom of 12 C isotope12

~" The term relative formula mass is used for all ionic and giant molecularcompounds, which do not form simple molecular entities.

~" Mr can be simply taken to be the sum of the Ar of all the atoms shown in theformula unit of the compound; the charge on the ion is ignored.

e.g. Mr of CO32-= 12.0+3(16.0) = 60.0

[NB. A formula unit refers to the smallest group of atoms from which the formulaof an ionic (or giant molecular) compound is determined.]

All the relative masses, both Arand Mr, have no units since they are ratios.

The molar mass of a substance is the mass of one mole of that substance.

Numerically, molar mass = Ar (or Mr) but has units of g mo1-1.

e.g. molar mass of CaCO3 - [40.1 + 12.0 + 3(16.0)] g mo1-1= 100.1 g mo1-1

© Step-by-Step

RelativeMolecular

Mass

RelativeFormula

Mass

Advanced Guide - Chemistry

Page 16: csonn t1 atoms, molecules and stoichiometry

Relative masses - Important!

1.1 Atoms, Molecules & Stoichiometry

The relative isotopic mass, Ar, of a particular isotope is defined as the ratio of thei the mass of an atom of 12C isotope,mass of one atom of the isotope to ]2

expressed on the 12C scale.

mass of one atom of a certain isotopex mass of one atom of 12C isotope

1 2

RelativeIsotopic

Mass

The relative molecular mass, Mr, of a substance is defined as the ratio of the average

mass of one molecule of the substance to ~ the mass of an atom of

12C isotope, expressed on the 12C scale.

Mr = mass of one moleculeof the substance± x mass of one atom of ~2C isotope1 2

Mr can be simply taken to be the sum of the Ar of all the atoms shown in the formulaof the substance.

e.g. Mr ofCaCO3 = 40.1 + 12.0+ 3(16.0) = 100.1

[NB. Relative atomic masses, Ar, are given in the Periodic Table.]

The relative formula mass of an ionic compound, Mr, is defined as the ratio of thethe mass of an atom ofaverage mass of one formula unit of the compound to

12C isotope, expressed on the lZc scale.

Mr = mass of one formula unit of the compound_x x mass of. one atom of 12 C isotope12

~" The term relative formula mass is used for all ionic and giant molecularcompounds, which do not form simple molecular entities.

~" Mr can be simply taken to be the sum of the Ar of all the atoms shown in theformula unit of the compound; the charge on the ion is ignored.

e.g. Mr of CO32-= 12.0+3(16.0) = 60.0

[NB. A formula unit refers to the smallest group of atoms from which the formulaof an ionic (or giant molecular) compound is determined.]

All the relative masses, both Arand Mr, have no units since they are ratios.

The molar mass of a substance is the mass of one mole of that substance.

Numerically, molar mass = Ar (or Mr) but has units of g mo1-1.

e.g. molar mass of CaCO3 - [40.1 + 12.0 + 3(16.0)] g mo1-1= 100.1 g mo1-1

© Step-by-Step

RelativeMolecular

Mass

RelativeFormula

Mass

Advanced Guide - Chemistry

Page 17: csonn t1 atoms, molecules and stoichiometry

Isotopes

• Look for chlorine in the Periodic Table.

• No. of protons = ?

• No. of electrons = ?

• No. of neutrons = ?

Page 18: csonn t1 atoms, molecules and stoichiometry

Mass spectrometer• Used to determine the masses of elements and

their isotopes

MASS SPECTROMETER

A mass spectrometer consists of ... an ion source, an analyser and a detector.

Ion source • gaseous atoms are bombarded by electrons from a gun and are IONISED• sufficient energy is given to form ions of 1+ charge• resulting ions can be ACCELERATED out of the ion source by an electric field

Analyser • charged particles will be DEFLECTED by a magnetic or electric field• the radius of the path depends on the value of their mass/charge ratio (m/z)• ions of heavier isotopes with larger m/z values follow a larger radius curve• as most ions are singly charged (1+), the path depends on their mass• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

Detector • by electric or photographic DETECTION methods• mass spectra record the mass/charge values and

relative abundance of each ion

4 Atomic StructureF321

© KNOCKHARDY PUBLISHING 2008

22Ne

21Ne

20NeANALYSER

ELECTRON GUN& ION SOURCE

DETECTOR

mass to charge ratio m/z

75%

25%

4 6 8 10 12

Abu

ndan

ce

75%

25%

2+ ion+ ion

Changes when the ion is 2+

• M/Z VALUE IS HALVED

• ABUNDANCE IS THE SAME

Page 19: csonn t1 atoms, molecules and stoichiometry

Mass spectrometerIon source

• gaseous atoms bombarded by electrons and ionised => 1+ charge ions formed

• resulting ions accelerated out of ion source by electric field

Analyser

• charged particles deflected by a magnetic or electric field

• radius of path depends on m/z ratio

• r = mv/Bq => lighter ions get deflected more

Detector

• mass spectra record the: mass/charge (m/z x-axis) and relative abundance (y-axis)

Page 20: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses

• From the Periodic Table, look up chlorine.

• How many protons, neutrons and electrons?

• 18.5 neutrons - does that make sense?

• Actually, 35.5 is the relative atomic mass, NOT relative isotopic mass!

Page 21: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q1

2

9701/23/O/N/10© UCLES 2010

ForExaminer’s

Use

Answer all the questions in the space provided.

1 The element magnesium, Mg, proton number 12, is a metal which is used in many alloys which are strong and light.

Magnesium has several naturally occurring isotopes. (a) What is meant by the term isotope?

..........................................................................................................................................

..........................................................................................................................................

...................................................................................................................................... [2]

(b) Complete the table below for two of the isotopes of magnesium.

isotopenumber of

protonsnumber of neutrons

number of electrons

24Mg

26Mg

[2]

A sample of magnesium had the following isotopic composition: 24Mg, 78.60%; 25Mg, 10.11%; 26Mg, 11.29%.

(c) Calculate the relative atomic mass, Ar, of magnesium in the sample. Express your answer to an appropriate number of significant figures.

[2]

Page 22: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q1

Page 23: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q2A sample of chlorine containing isotopes of mass numbers 35 and 37 was analysed in a mass-spectrometer.

a) The relative abundance of 35Cl is 75.8% and of 37Cl is 24.2%. Calculate relative atomic mass of a chlorine atom.

b) How many peaks corresponding to Cl2+ were recorded? Identify them.

Page 24: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q2

Page 25: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q3The isotopic composition of an element is indicated below. What is the relative atomic mass of the element?

Page 26: csonn t1 atoms, molecules and stoichiometry

Relative atomic masses - Q3

Page 27: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant

1.1 Atoms, Molecules & Stoichiometry

o~ :Theterm ' mole' defined in termsof the A vogadroco~t~.: ~. ~: i; i .i-:.-.~. ~ :: ~"~ i

Owing to the small size of atoms, molecules and ions, properties of these species areoften compared on a mole basis.

M o l e The mole is the unit of the amount of substance.

One mole of a substance contains as many particles (or elementary entities) of thatsubstance as there are atoms of carbon in 12 grams (0.012 kg) of carbon-12.

What the particles are must be stated (e.g. atoms, molecules, ions, electrons).[given in "Signs, Symbols and Systematics (The ASE Companion to 16-19 Science, 2000)"]

AvogadroConstant

The Avogadro constant, L, may be taken as the number of carbon atoms in 12 g ofcarbon-12, and has been determined experimentally to have a value of about6.02 × 1023 mol-~ (3 sf). Equivalently,

One mole of any substance is the amount of substance containing a number ofparticles equal to the Avogadro constant (6.02 × 1023).

1 mol of 12C atoms - 6.02 × 1023 12C atoms

1 mol of H20 molecules - 6.02 × 1023 H20 molecules

1 mol of CO32- ions - 6.02 x 1023 CO32- ions1 mol of electrons - 6.02 × 1023 electrons

A mole of any substance has a mass in grams numerically equal to it's Ar or Mr.

e.g. mass of 1 molof ~2C atoms = 12.0 g

mass of 1 mol of H20 molecules = [2(1.0) + 16.0] g = 18.0 g

mass of 1 mol of CO32- ions = [ 12.0 + 3(16.0)] g = 60.0 g

Hence, a phrase such as '16 g of oxygen' is ambiguous.

It could mean 16 g of oxygen atoms (O), 16 g of oxygen molecules (O2)or 16 g of oxygen ions (02-).

It could also mean 16 g of 160 isotope or 16 g of 180 isotope.

Relationship between the number of moles (n), mass (m) and relative masses:

where m = mass in grams

Ar - relative atomic mass of elementMr = relative molecular mass of compound

Note: Strictly, number of moles n =mass m (in g)

molar mass M (in g mol-l)

Since numerically M= Mr, for simplicity in calculations, we use n -

Relationship between the number of particles, moles (n), and Avogadro constant (L)"

of particles = nL l where L = 6.02 x 1023 mol-~.no.

[Avogadro constant is sometimes given the symbol NA.]© Step-by-Step

Advanced Guide - Chemistry

Page 28: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q12

! UCLES 2004 9701/01/M/J/04

Section A

For each question there are four possible answers, A, B, C, and D. Choose the one you consider to

be correct.

1 Which of these samples of gas contains the same number of atoms as 1g of hydrogen

(Mr : H2, 2)?

A 22 g of carbon dioxide (Mr : CO2, 44)

B 8 g of methane (Mr : CH4, 16)

C 20 g of neon (Mr : Ne, 20)

D 8 g of ozone (Mr : O3, 48)

2 Self-igniting flares contain Mg3P2. With water this produces diphosphane, P2H4, which is

spontaneously flammable in air.

Which equation that includes the formation of diphosphane is balanced?

A Mg3P2 + 6H2O ! 3Mg(OH)2 + P2H4

B Mg3P2 + 6H2O ! 3Mg(OH)2 + P2H4 + H2

C 2Mg3P2 + 12H2O ! 6Mg(OH)2 + P2H4 + 2PH3

D 2Mg3P2 + 12H2O ! 6Mg(OH)2 + 3P2H4

3 Use of the Data Booklet is relevant to this question.

Most modern cars are fitted with airbags. These work by decomposing sodium azide to liberate

nitrogen gas, which inflates the bag.

2NaN3 ! 3N2 + 2Na

A typical driver’s airbag contains 50 g of sodium azide.

Calculate the volume of nitrogen this will produce at room temperature.

A 9.2 dm3

B 13.9 dm3

C 27.7 dm3

D 72.0 dm3

4 What is the order of increasing energy of the listed orbitals in the atom of titanium?

A 3s 3p 3d 4s

B 3s 3p 4s 3d

C 3s 4s 3p 3d

D 4s 3s 3p 3d

Page 29: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q1

Page 30: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q2

14. A fullerene molecule consists of 60 carbonatoms.

Approximately how many such molecules arepresent in 12 g of this type of carbon?

A 1.0 × 1022

B 1.2 × 1023

C 6.0 × 1023

D 3.6 × 1025

15. Avogadro’s Constant is the same as thenumber of

A molecules in 16.0 g of oxygen

B atoms in 20.2 g of neon

C formula units in 20.0 g of sodiumhydroxide

D ions in 58.5 g of sodium chloride.

16. The equation for the complete combustion ofpropane is:

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(ℓ)

30 cm3 of propane is mixed with 200 cm3 ofoxygen and the mixture is ignited.

What is the volume of the resulting gasmixture? (All volumes are measured at thesame temperature and pressure.)

A 90 cm3

B 120 cm3

C 140 cm3

D 210 cm3

17. A mixture of carbon monoxide and hydrogencan be converted into water and a mixture ofhydrocarbons.

nCO + (2n + 1)H2 → nH2O + hydrocarbons

What is the general formula for thehydrocarbons produced?

A CnH2n–2

B CnH2n

C CnH2n+1

D CnH2n+2

Page four

9. Which of the following chlorides is most likelyto be soluble in tetrachloromethane, CCl4?

A Barium chloride

B Caesium chloride

C Calcium chloride

D Phosphorus chloride

10. Which of the following compounds exists asdiscrete molecules?

A Sulphur dioxide

B Silicon dioxide

C Aluminium oxide

D Iron(II) oxide

11. An element (melting point above 3000 °C)forms an oxide which is a gas at roomtemperature.

Which type of bonding is likely to be presentin the element?

A Metallic

B Polar covalent

C Non-polar covalent

D Ionic

12. Which of the following compounds has polarmolecules?

A CO2

B NH3

C CCl4D CH4

13. How many moles of oxygen atoms are in 0.5 mol of carbon dioxide?

A 0.25

B 0.5

C 1

D 2

[X012/301]

Page 31: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q2

Page 32: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q3

Nickel makes up 20% of the total mass of a coin. The coin has a mass of 10.0 g.

How many nickel atoms are in the coin?

Page 33: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q3

Page 34: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q4

Analytical chemists can detect very small amounts of amino acids, down to 3 x 10-21 mol.

How many molecules of an amino acid (Mr = 200) would this be?

Page 35: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q4

Page 36: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q5

A fullerene molecule consists of 60 carbon atoms. Approximately how many such molecules are present in 12 g of this type of carbon?

Page 37: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q5

Page 38: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q6

The mass of 1 mole of sodium is 23 g. What is the approximate mass of one sodium atom?

Page 39: csonn t1 atoms, molecules and stoichiometry

Mole concept and the Avogadro’s constant - Q6

Page 40: csonn t1 atoms, molecules and stoichiometry

Empirical & molecular formulae

r . . . . . 111 Atoms, Molecules & Stoichiometry 13

~,,,, ,. _, ......

4, " Definition .Of th terms empirical ahd mrlecidar fo~lae~ : i i?~ ;: ? '-:':i" ~;:~ : ;i.:i:-; ;;: !":i)5 i: . i~ ,'- ~ :.~ Caicu!atiOn. of empirical and molecular formulae from combustion data or

2-..composit ionbymass,. . . . . . . . . . , . :~:. .- :~.~J... . . ' . - . : i -~.. :~:: . . . . - . . . :-~: . . ;

~ . Moi¢~ular formula de~:ived from empirica!fo_rm.u_!a._ _qnfi re..!ative m._o_le~la.rZ.~...~i, . . i

[The empirical formula of a compound is the simplest formula which shows the ratioof the atoms of the different elements in the compound.

]The molecular formula of a compound is one which shows the actual number of

[atoms of each element present in one molecule of a compound.

The molecular formula can be obtained if the empirical formula and Mr are known.

EmpiricalFormula

MolecularFormula

Steps to follow to calculate empirical formula:1. Change percentage composition to mass, m (in g).

m2 . Divide the mass (m) of each element by its respective relative n =

atomic mass (AO to get the number of moles of each.

3 . Divide the number of moles of each element by the smallest number of moles.This usually gives whole number ratios of moles.

If a fraction still exists,

(a) change the decimal portion to a fraction,

(b) change compound fractions to simple fractions, and

(c) multiply all numbers obtained in step 3 by the denominator of the fraction.7 . 1 = 7 . 2e.g. ratio 3.5" 1 = 5-

F r o m

Compositionby Mass

e.g. To calculate the empirical formula given the following composition by mass:

(i) compound A: 12.8 % carbon, 2.1% hydrogen and 85.1% bromine

C H B r

mass / g 12.8 2.1 85.1

no. of mole

simplest ratio

12.8 _ 1.0712

85.1_ 1.068 0

.'. mole ratio C:H:Br = 1"2" 1

Hence, empirical formula of compound A is CH2Br.

(ii) compound B: 48.8 % carbon, 13.5 % hydrogen and 37.7 % nitrogen

C H N

mass / g 48 .8 13 .5 37 .7.......

no. of mole,.

simplest ratio

48.8 _ 4.0712

13.5 _ 13.51

3 7 . 7 - 2 . 6 914

mole ratio C:H:N= 1.5"5" 1- 3 . 5 . 1

= 3 " 1 0 " 2

Hence, empirical formula of compound B is C3HtoN2.

Advanced Guide - Chemistry

© Step-by-Step

Page 41: csonn t1 atoms, molecules and stoichiometry

Empirical formula - Q1

4

© UCLES 2013 9701/13/M/J/13

6 Which least resembles an ideal gas at room temperature and pressure?

A ammonia

B helium

C hydrogen

D methane

7 Use of the Data Booklet is relevant to this question.

When 0.15 g of an organic compound is vaporised, it occupies a volume of 65.0 cm3

at 405 K and

1.00 × 105

Nm–2

.

Using the expression pV = nRT, which of the following expressions should be used to calculate

the relative molecular mass, Mr, of the compound?

A

40531.8

101106515.056

×

××××−

B

35

1065101

40531.815.0

×××

××

C

40531.8

101106515.053

×

××××−

D

65

1065101

40531.815.0

×××

××

8 Which compound is the only gas at room temperature and pressure?

A CH3CH2CH2NH2 Mr = 59.0

B CH3CH2CH2OH Mr = 60.0

C CH2OHCH2OH Mr = 62.0

D CH3CH2Cl Mr = 64.5

9 Which formula represents the empirical formula of a compound?

A CH4O B C2H4 C C6H12 D H2O2

10 Use of the Data Booklet is relevant to this question.

A washing powder contains sodium hydrogencarbonate, NaHCO3, as one of the ingredients. In a

titration, a solution containing 1.00 g of washing powder requires 7.15 cm3

of 0.100 mol dm–3

sulfuric acid for complete reaction. The sodium hydrogencarbonate is the only ingredient that

reacts with the acid.

What is the percentage by mass of sodium hydrogencarbonate in the washing powder?

A 3.0 B 6.0 C 12.0 D 24.0

Page 42: csonn t1 atoms, molecules and stoichiometry

Empirical formula - Q1

Page 43: csonn t1 atoms, molecules and stoichiometry

Empirical formulae - Q2

In leaded petrol, there is an additive composed of lead, carbon and hydrogen only. This compound contains 29.7 % carbon and 6.19 % hydrogen by mass.

What is the value of x in the empirical formula PbC8Hx?

Page 44: csonn t1 atoms, molecules and stoichiometry

Empirical formulae - Q2

Page 45: csonn t1 atoms, molecules and stoichiometry

Molecular formulae - Q1

5

© UCLES 2010 9701/11/O/N/10 [Turn over

12 Camphor is a white solid which was used to make the early plastic celluloid. Camphor contains

the same percentage by mass of hydrogen and oxygen.

What is the molecular formula of camphor?

A C10H6O6 B C10H8O C C10H16O D C10H10O2

13 Why is the first ionisation energy of phosphorus greater than the first ionisation energy of silicon?

A A phosphorus atom has one more proton in its nucleus.

B The atomic radius of a phosphorus atom is greater.

C The outer electron in a phosphorus atom is more shielded.

D The outer electron in a phosphorus atom is paired.

14 When magnesium nitrate, Mg(NO3)2.7H2O, is heated, which three gases are given off?

A dinitrogen oxide, oxygen, water vapour

B hydrogen, nitrogen, oxygen

C hydrogen, nitrogen dioxide, oxygen

D nitrogen dioxide, oxygen, water vapour

15 Ammonium sulfate in nitrogenous fertilisers in the soil can be slowly oxidised by air producing

sulfuric acid, nitric acid and water.

How many moles of oxygen gas are needed to oxidise completely one mole of ammonium

sulfate?

A 1 B 2 C 3 D 4

16 Chile saltpetre, NaNO3, contains sodium iodide as an impurity.

Aqueous silver nitrate is added to an aqueous solution of Chile saltpetre. Concentrated aqueous

ammonia is then added.

Which observations are made?

with acidified silver

nitrate

with concentrated

aqueous ammonia

A no precipitate no further reaction

B no precipitate precipitate forms

C precipitate forms precipitate dissolves

D precipitate forms precipitate remains

Page 46: csonn t1 atoms, molecules and stoichiometry

Molecular formulae - Q1

Page 47: csonn t1 atoms, molecules and stoichiometry

Molecular formulae - Q2

8806-6104

– 3 –

Turn over

N06/4/CHEMI/SPM/ENG/TZ0/XX+

1. The empirical formula of a compound is C H O2 4 . Which molecular formulas are possible for this compound?

I. CH COOH3

II. CH CH CH COOH3 2 2

III. CH COOCH CH3 2 3

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

2. Calcium carbonate decomposes on heating as shown below.

CaCO CaO CO3 2

When 50 g of calcium carbonate are decomposed, 7 g of calcium oxide are formed. What is the percentage yield of calcium oxide?

A. 7 %

B. 25 %

C. 50 %

D. 75 %

3. Sodium reacts with water as shown below.

__ __ __ __Na H O NaOH H2 2

What is the total of all the  coefficients  when   the  equation   is  balanced  using   the  smallest  possible  whole numbers?

A. 3

B. 4

C. 6

D. 7

Page 48: csonn t1 atoms, molecules and stoichiometry

Molecular formulae - Q2

Page 49: csonn t1 atoms, molecules and stoichiometry

Empirical & molecular formulae

9

5070/2/O/N/03 [Turn over

A7 Phosphorus is a non-metal.This diagram shows the structure of one molecule of phosphorus(III) oxide.

(a) (i) Give the molecular formula of phosphorus(III) oxide.

...................................................................................................................................

(ii) Give the empirical formula of phosphorus(III) oxide.

...............................................................................................................................[2]

(b) Explain why phosphorus(III) oxide has the properties given below.

Property 1 Phosphorus(III) oxide is acidic

explanation .......................................................................................................................

..........................................................................................................................................

Property 2 Phosphorus(III) oxide has a low melting point.

explanation .......................................................................................................................

..........................................................................................................................................

Property 3 Phosphorus(III) oxide will not conduct electricity when molten.

explanation .......................................................................................................................

......................................................................................................................................[3]

O

P

ForExaminer’s

Use

www.xtremepapers.net

Nov 04, (2)

Page 50: csonn t1 atoms, molecules and stoichiometry

Mole concept - using mass

• For pure substances (elements, compounds) with given mass,

mole = mass/molar mass [UNITS?]

• Also note that

density = mass/volume [UNITS?]

Page 51: csonn t1 atoms, molecules and stoichiometry

Mole concept - using concentration

• Solutions refer to substances dissolved in water.

• What is the unit of concentration? definition?

• Given concentration and volume,

mole = concentration (mole/dm3) * volume (dm3)

mole = mass concentration (g/dm3) / Mr (g/mole)

Page 52: csonn t1 atoms, molecules and stoichiometry

Mole concept - using molar volume of gases

• What is molar volume? unit?

• Conditions?

• Room temperature and pressure (r.t.p.)

=> 25°C (298K), 1 atmosphere, molar volume = 24.0 dm3/mole

• Standard temperature and pressure (s.t.p.)

=> 0°C (273K), 1 atmosphere, molar volume = 22.4 dm3/mole

• Given volume of gases, at r.t.p.

• mole = volume (dm3) / 24 (dm

3/mole)

• mole = volume (cm3) / 24 000 (cm

3/mole)

Page 53: csonn t1 atoms, molecules and stoichiometry

Mole concept - Q1What is the number of molecules in 500 cm3 of oxygen under room conditions?

Page 54: csonn t1 atoms, molecules and stoichiometry

Mole concept - Q2

2

© UCLES 2008 9701/01/O/N/08

Section A

For each question there are four possible answers, A, B, C, and D. Choose the one you consider to

be correct.

1 Use of the Data Booklet is relevant to this question.

Titanium(IV) oxide, TiO2, is brilliantly white and much of the oxide produced is used in the

manufacture of paint.

What is the maximum amount of TiO2 obtainable from 19.0 tonnes of the ore ilmenite, FeTiO

3?

A 10.0 tonnes B 12.7 tonnes C 14.0 tonnes D 17.7 tonnes

2 Carbon disulphide vapour burns in oxygen according to the following equation.

CS2(g) + 3O

2(g) → CO

2(g) + 2SO

2(g)

A sample of 10 cm3

of carbon disulphide was burned in 50 cm3

of oxygen. After measuring the

volume of gas remaining, the product was treated with an excess of aqueous sodium hydroxide

and the volume of gas measured again. All measurements were made at the same temperature

and pressure, under such conditions that carbon disulphide was gaseous.

What were the measured volumes?

volume of gas

after burning / cm3

volume of gas after

adding NaOH(aq) / cm3

A 30 0

B 30 20

C 50 20

D 50 40

3 In which pair do both atoms have one electron only in an s orbital in their ground states?

A Ca, Sc B Cu, Be C H, He D Li, Cr

4 Use of the Data Booklet is relevant to this question.

Hard water contains calcium ions and hydrogencarbonate ions arising from dissolved calcium

hydrogencarbonate, Ca(HCO3)2.

How many electrons are present in the hydrogencarbonate anion?

A 30 B 31 C 32 D 33

Page 55: csonn t1 atoms, molecules and stoichiometry

Mole concept - Q3

2

© UCLES 2011 9701/11/O/N/11

Section A

For each question there are four possible answers, A, B, C, and D. Choose the one you consider to

be correct.

1 Three elements, X, Y and Z, have the physical properties shown in the table.

element melting point

/ °C

boiling point / °C

density

/ g cm-3

X –7 59 3.12

Y 98 883 0.97

Z 649 1107 1.74

What could be the identities of X, Y and Z?

X Y Z

A Br2 Al Si

B Br2 Na Mg

C I2 Mg Na

D I2 Si K

2 Use of the Data Booklet is relevant to this question.

Lead(IV) chloride will oxidise bromide ions to bromine. The Pb4+

ions are reduced to Pb2+

ions in

this reaction.

If 6.980 g of lead(IV) chloride is added to an excess of sodium bromide solution, what mass of

bromine would be produced?

A 0.799 g B 1.598 g C 3.196 g D 6.392 g

3 Which element has an equal number of electron pairs and of unpaired electrons within orbitals of

principal quantum number 2?

A beryllium

B carbon

C nitrogen

D oxygen

Page 56: csonn t1 atoms, molecules and stoichiometry

Mole concept - Q3

Page 57: csonn t1 atoms, molecules and stoichiometry

Percentage by mass - Q1

4

© UCLES 2013 9701/13/M/J/13

6 Which least resembles an ideal gas at room temperature and pressure?

A ammonia

B helium

C hydrogen

D methane

7 Use of the Data Booklet is relevant to this question.

When 0.15 g of an organic compound is vaporised, it occupies a volume of 65.0 cm3

at 405 K and

1.00 × 105

Nm–2

.

Using the expression pV = nRT, which of the following expressions should be used to calculate

the relative molecular mass, Mr, of the compound?

A

40531.8

101106515.056

×

××××−

B

35

1065101

40531.815.0

×××

××

C

40531.8

101106515.053

×

××××−

D

65

1065101

40531.815.0

×××

××

8 Which compound is the only gas at room temperature and pressure?

A CH3CH2CH2NH2 Mr = 59.0

B CH3CH2CH2OH Mr = 60.0

C CH2OHCH2OH Mr = 62.0

D CH3CH2Cl Mr = 64.5

9 Which formula represents the empirical formula of a compound?

A CH4O B C2H4 C C6H12 D H2O2

10 Use of the Data Booklet is relevant to this question.

A washing powder contains sodium hydrogencarbonate, NaHCO3, as one of the ingredients. In a

titration, a solution containing 1.00 g of washing powder requires 7.15 cm3

of 0.100 mol dm–3

sulfuric acid for complete reaction. The sodium hydrogencarbonate is the only ingredient that

reacts with the acid.

What is the percentage by mass of sodium hydrogencarbonate in the washing powder?

A 3.0 B 6.0 C 12.0 D 24.0

Page 58: csonn t1 atoms, molecules and stoichiometry

Percentage by mass - Q2

2.0 g of an iron wire was dissolved in excess dilute sulphuric acid and the solution was made up to 200 cm3. 25.0 cm3 of this solution needed 26.45 cm3 of a 0.0188 mol dm-3 potassium manganate (VII) solution for oxidation.

Calculate the percentage of iron in the iron wire.

Page 59: csonn t1 atoms, molecules and stoichiometry

Percentage by mass - Q2

Page 60: csonn t1 atoms, molecules and stoichiometry

Percentage by mass - Q2

Page 61: csonn t1 atoms, molecules and stoichiometry

Limiting and excess reactant

• Excess reactant - not completely used up at the end of reaction

• Limiting reagent - completely used up at the end of the reaction - it determines the yield of the reaction

Page 62: csonn t1 atoms, molecules and stoichiometry

Percentage yield

• The percentage yield is a measure of the efficiency of the reaction.

• % yield = actual yield/theoretical yield x 100%

• actual yield uses mole value for limiting reactant

Page 63: csonn t1 atoms, molecules and stoichiometry

Limiting and excess reactant - Q1

[X012/12/02] Page three

1. Isotopes of an element have

A the same mass number

B the same number of neutrons

C equal numbers of protons and neutrons

D different numbers of neutrons.

2. Four metals W, X, Y and Z and their compounds behaved as described.

(i) Only X, Y and Z reacted with dilute hydrochloric acid.

(ii) The oxides of W, X and Y were reduced to the metal when heated with carbon powder. The oxide of Z did not react.

(iii) A displacement reaction occurred when X was added to an aqueous solution of the nitrate of Y.

What is the correct order of reactivity of these metals (most reactive first)?

A W, Y, X, Z

B W, X, Y, Z

C Z, X, Y, W

D Z, Y, X, W

3. A positively charged particle with electron arrangement 2, 8 could be

A a neon atom

B a fluoride ion

C a sodium atom

D an aluminium ion.

4. A solution of potassium carbonate, made up using tap water, was found to be cloudy.

This could result from the tap water containing

A sodium ions

B chloride ions

C magnesium ions

D sulphate ions.

(You may wish to refer to the Data Booklet.)

5. 1 mol of hydrogen gas and 1 mol of iodine vapour were mixed and allowed to react. After t seconds, 0·8 mol of hydrogen remained.

The number of moles of hydrogen iodide formed at t seconds was

A 0·2

B 0·4

C 0·8

D 1·6.

6. In a reaction involving gases, an increase in temperature results in

A an increase in activation energy

B an increase in the enthalpy change

C a decrease in the activation energy

D more molecules per second forming an activated complex.

7. Calcium carbonate reacts with nitric acid as follows.

CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(ℓ) + CO2(g)

0·05 mol of calcium carbonate was added to a solution containing 0·08 mol of nitric acid.

Which of the following statements is true?

A 0·05 mol of carbon dioxide is produced.

B 0·08 mol of calcium nitrate is produced.

C Calcium carbonate is in excess by 0·01 mol.

D Nitric acid is in excess by 0·03 mol.

[Turn over

Page 64: csonn t1 atoms, molecules and stoichiometry

Limiting and excess reactant - Q1

Page 65: csonn t1 atoms, molecules and stoichiometry

Limiting and excess reactant - Q2

10

© UCLES 2008 9701/01/O/N/08

28 Bees use 2-methylbutyl ethanoate as an ‘alarm’ pheromone. When disturbed, individual bees on

guard will raise their abdomen and emit the alarm pheromone, fanning their wings to aid its

dispersal. This alerts other bees to a danger and makes them ready to sting when required.

O

O

CC

C

C

H

H

H H

C

H

H

C

H

H

H

H

HH C

H

H

2-methylbutyl ethanoate

Which starting materials would be required to synthesise 2-methylbutyl ethanoate?

A CH3CH2OH and CH3CH2CH(CH3)CO2H

B CH3CO2H and CH3CH2CH(CH3)CH2OH

C CH3CH2OH and CH3CH2CH(CH3)CH2CO2H

D CH3CO2H and CH3CH2CH(CH3)CH2CO2H

29 The product of the reaction between propanone and hydrogen cyanide is hydrolysed under acidic

conditions.

What is the formula of the final product?

A CH3CH(OH)CO2H

B CH3CH2CH2CO2H

C (CH3)2CHCONH2

D (CH3)2C(OH)CO2H

30 Use of the Data Booklet is relevant to this question.

Ethyl ethanoate can be obtained from ethanoic acid and ethanol by the following reaction.

CH3CH2OH + CH3CO2H CH3CO2CH2CH3 + H2O

Ethanol (30 g) and ethanoic acid (30 g) are heated under reflux together, and 22 g of ethyl

ethanoate are obtained.

What is the yield of the ester?

A 25 % B 38 % C 50 % D 77 %

Page 66: csonn t1 atoms, molecules and stoichiometry

Limiting and excess reactant - Q2

Page 67: csonn t1 atoms, molecules and stoichiometry

Common reactions

• Acid-base reactions

• Redox reactions

• Precipitation reactions

Page 68: csonn t1 atoms, molecules and stoichiometry

Balancing equations - Q1

Ammonium sulphate in nitrogeneous fertiliser in the soil can be slowly oxidised by air producing sulphuric acid, nitric acid and water.

Page 69: csonn t1 atoms, molecules and stoichiometry

Balancing equations - Q2

The petrol additive tetraethyl-lead(IV), Pb(C2H5)4, is now banned in many countries. When it is completely burned in air, lead(II) oxide, CO2 and H2O are formed.

Page 70: csonn t1 atoms, molecules and stoichiometry

Conservation of mass

During any physical or chemical change, the total mass of the products remains equal to the total mass of the reactants (in a closed system - no products escape off)

Page 71: csonn t1 atoms, molecules and stoichiometry

Conservation of mass - Q1Given this reaction,

HgO (s) —> Hg (l) + ½O2 (g)

If you start with 100 g of HgO and you end up with 92.6 g of Hg, what is the mass of oxygen formed?

Page 72: csonn t1 atoms, molecules and stoichiometry

Conservation of mass - Q2

Calcium sulphate can exist in the hydrated form of gypsum, CaSO4.2H2O. When it is heated, gypsum loses some water to form plaster of Paris, which is used to make plaster casts.

If 100 g of gypsum loses 15.6 g of water, find the formula of plaster of Paris.

Page 73: csonn t1 atoms, molecules and stoichiometry

Conservation of mass - Q2

Page 74: csonn t1 atoms, molecules and stoichiometry

Conservation of mass - Q3

A 5.00 g sample of an anhydrous Group II metal nitrate loses 3.29 g in mass on strong heating.

Identify the Group II metal present.

N.B. Group II metal nitrate decomposes to give metal oxide, nitrogen dioxide and oxygen.

Page 75: csonn t1 atoms, molecules and stoichiometry

Conservation of mass - Q3

Page 76: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion

• What is hydrocarbon?

• What are the products of complete combustion?

Page 77: csonn t1 atoms, molecules and stoichiometry

Complete combustion of hydrocarbons

UsingCombustion

Data

1.1 Atoms, Molecules & Stoichiometry

The molecular formula of hydrocarbons can be determined by combustion in excessoxygen (to form carbon dioxide and steam). A gaseous hydrocarbon, CxHy, explodeswith excess 02 according to the general equation.

Y Y H 2 0C x H y + ( x + - ~ ) 0 2 - ~ x C O 2 + ~ -

Molecular formula is a multiple of empirical formula, and can be determined givenboth the empirical formula and the mass of one mole of a compound (or Mr value).

e.g. To calculate the molecular formula given the following combustion data:

(i) 10 cm3 of a gaseous hydrocarbon C required 20 cm3 of oxygen for completecombustion. 10 cm3 of carbon dioxide was produced.[All gases were measured under the same conditions.]

Y Y H 2 0C x H y J r - ( X d - - ~ ) O 2 ~ x C O 2 q - " ~ "

10 cm3 20 cm3 10 cm31 mol 2 mol 1 mol

y=:> x= l a n d ( x + - ~ ) = 2 o r y = 4

Hence, molecular formula of hydrocarbon C is CH4.

(ii) 150 cm3 of oxygen were added to 20 cm3 of a gaseous hydrocarbon D.After explosion and cooling, the gaseous mixture occupied 130 cm3 and, afterabsorption by potassium hydroxide, 90 cm3 of oxygen remained.[All volumes being measured at r.t.p.]

Y Y H 2 0G H y + ( x + 7 ) O z ~ x C O 2 + 5 -2 0 c m 3 ( 1 5 0 - 9 0 ) (130-90) [KOH, an alkali, is used to remove CO2.]

= 60 cm3 = 40 cm31 mol 3 mol 2 mol

yx = 2 a n d ( x + ~ - ) = 3 o r y = 4

Hence, molecular formula of hydrocarbon D is C2H4.

(iii) Complete combustion of a hydrocarbon E (Mr 78) yields 2.64 g of carbondioxide and 0.540 g of water.

Y Y H20° CxHy + (x+- ; )O2 ~ x CO2 +7

tool of C in hydrocarbon = mol of CO2= 2.64 _ 2.64 _ 0.0600 tool

1 2 . 0 + 2 ( 1 6 . 0 ) 4 4 . 0

tool of H in hydrocarbon = 2 x mol of H20

= 2 x ( 0.540 -- 1.082(1.0)+16.0 )aS.0 -- 0.0600 mol.'. mole ratio C:H = 1"1

Hence, empirical formula of hydrocarbon E is CH.

Let the molecular formula of hydrocarbon E be C,H,.

n(12) + n(1) = 7813n - 78

n - 1 2 ~ = 6

.'. molecular formula of hydrocarbon E is C6H6.

© Step-by-Step

Advanced Guide - Chemistry

Page 78: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion• This is the general equation for combustion of hydrocarbon

only, (remember by heart)

CxHy + (x+y/4)O2 —> xCO2 + y/2H2O

• With many of these calculation questions, make the following assumptions,

a) vol. of gases = no. of mole of gases

b) vol. of liquid water = 0

Page 79: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q1

0.200 mole of a hydrocarbon undergo complete combustion to give 35.2 g of carbon dioxide and 14.4 g of water as the only products.

What is the molecular formula of the hydrocarbon?

Page 80: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q1

Page 81: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q2

10 cm3 of gaseous hydrocarbon required 20 cm3 of oxygen gas for complete combustion under standard temperature and pressure (STP). 10 cm3 of carbon dioxide was produced.

What is the molecular formula of the hydrocarbon and hence it’s Mr?

Page 82: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q2

Page 83: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q3

What volume of oxygen, at room temperature and pressure, is needed for complete combustion of 0.1 mol of ethanol?

Page 84: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q3

Page 85: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q4

A pure hydrocarbon is used in bottled gas for cooking and heating. When 10 cm3 of the hydrocarbon is burned in 70 cm3 of oxygen (an excess), the final gaseous mixture contains 30 cm3 of carbon dioxide and 20 cm3 of unreacted oxygen.

All gaseous volumes were measured under identical conditions.

What is the formula of the hydrocarbon?

Page 86: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q4

Page 87: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q5

Carbon disulphide vapour burns in oxygen according to the following equation.

CS2 (g) + 3O2 (g) —> CO2 (g) + 2SO2 (g)

A sample of 10 cm3 of carbon disulphide was burned in 50 cm

3 of o x y g e n .

After measuring the volume of gas remaining, the product was treated with an excess of aqueous sodium hydroxide and the volume of gas measure again. All measurements were made at the same temperature and pressure, under such conditions that carbon disulphide was gaseous.

What were the measured volumes of gas, in cm3,

(a) after burning, and

(b) after adding NaOH (aq)?

Page 88: csonn t1 atoms, molecules and stoichiometry

Hydrocarbon combustion - Q5

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Hydrocarbon combustion - Q6

20 cm3 of a gaseous hydrocarbon was burned in 150 cm3 of O2 at standard temperature and pressure (STP).

The gaseous products have a total volume of 130 cm3. When this product is passed over soda lime, the volume of the product is decreased to 90 cm3 (Soda lime is used to remove CO2).

What is the molecular formula of the hydrocarbon?

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Hydrocarbon combustion - Q6

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Isotopes - Q1

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Isotopes - Q2

2208-6108

– 4 – M08/4/CHEMI/HP2/ENG/TZ1/XX

2. (a) The mass spectrometer is used to investigate the isotopic composition of elements. Thallium has two isotopes 81

203Tl and 81205Tl.

(i) State the symbols of the two singly charged ions that would form in the mass spectrometer.

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[1]

(ii) State which ion will follow the path marked X on the diagram. [1]

detector

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(iii) Some doubly charged ions form in the mass spectrometer. Suggest with a reason whether  they  would  be  deflected  less  than  or  more  than  the  ions  at  X and Y.

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[2]

(b) Naturally occurring boron consists of the two isotopes, 10 B and 11B. The relative atomic mass of boron is 10.81. Determine the percentage abundance of these isotopes.

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[2]

X

Y

magnetic  field

0413

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Isotopes - Q3