SECTION 1 (7%): ATOMS, MOLECULES AND STOICHIOMETRY

• Uses of isotopes (e.g. radioactive dating & leak detection)

• Mass spectrometer (e.g. determination of relative atomic or molecular mass from relative intensities of peaks in mass spectrum)

• Electrolysis (e.g. calculation involving volume of gas evolved / mass of solid deposited / thickness of electroplating layer, oxidation states of metals)

• Ideal gas equation (e.g. calculation of partial pressure & densities for a mixture of gases at 2 different temperature)

• Calculation involving volumetric analysis / back titration (e.g. determination of % by mass of active chlorine in bleaching solution, % of mass of N in ammonium fertilizer)

• Balancing nuclear reactions

1.1 THE ATOMIC STRUCTURE

• Relative atomic mass:

• Atomic number: equal to the number of p_____ in the nucleus

• Mass number: number of protons plus no. of neu_____s

• Isotopes: e.g. 12C, 13C, 14C; 35Cl, 37Cl

1.2 RADIOACTIVITY

• Radioactivity: the disinte________ of certain unstable nuclei with emission of ra______

• 3 types of radiation from radioactive substance

1. ray ( particle)

2. ray (particle)

3. ray

• Nuclear equations: all the following equations which involve changes in the number of neutrons and / or protons are known as nuclear equations. (balancing in both mass and atomic number on both sides) e.g. 9Be + 4He 12C + 1n

9Be + 1H 10B +

baX+d

cY feZ

b + d = fa + c = e

decay: mass no.-4, atomic no. –2 decay: mass no. no change, atomic no. +1

• Half life:

the time for the radiation to be halved, or for the radio-nuclei to be reduced to half.

• Radioactive decay:

ln Io / It = kt & k = ln 2 / t1/2

(t1/2 : half life)

Uses of radioactivity

• Leak detection

• radiotherapy: ray in cancer treatment

• nuclear power

• tracers: for studying metabolism in living organism

• carbon-14 dating

1.3 RELATIVE ISOTORIC, ATOMIC AND MOLECULAR

MASSES

• Isotope: atoms with the s______ number of protons but dif_____ numbers of neutrons.

• Relative atomic mass:e.g. relative atomic mass of Cl atom = 0.75 * 35 + 0.25 * 37 = 35.5

• Relative molecular mass: e.g. the relative molecular mass of water H2O = (2 * at. mass H) +(at. mass O)

• Mass spectrometer: used for the determination of relative masses of particles. It responds to the mass-charge ratio.

NOT mass!

• Mass spectrum:

(i) y-axis peak height (or relative intensity) is directly proportional to the % abundance of the ionized species (e.g. 14N+ or CO2

+),

(ii) x-axis m/e ratio

1.4 THE MOLE CONCEPT, P-V-T RELATIONSHIPS OF GASES

• A mole: the amount of a substance (6.02 * 1023).

• Avogadro’s law: equal volumes of gases at same conditions equal no. of moles.

• Molar volume of gas at s.t.p.: 22.4 dm3 in volume.

• Ideal gas equation: PV = nRT

• Relationship between density & molar mass: the ideal gas equation can be rewritten for the calculation of the gas density (, in g m-3 [or kg m-3]). For a gas of mass m (in g [or kg]) & a molar mass of M (in g mol-1 [or kg mol-1]), a volume of V (in m3) at a pressure P (Nm-2, Pa):

PV =nRT & =m/V PV = (m/M)RT P = (/M)RT

• Partial Pressure: in a gas mixture, the partial pressure of each component gas is the pressure that the gas would exert if it were the only gas occupying the total volume at that temperature.

• Dalton’s Law of Partial Pressure: for three gases 1, 2 and 3 in mixture P total = P1+ P2 + P3

rewritten as P total = (n1+n2+n3)(RT/V) = n total (RT/V)

• Mole fraction:

the mole fraction (I) of a component in a mixture is defined as the no. of moles of that component over the total no. of moles of gases,

i.e. 1 = n1/(n1+n2+n3) = n1/ntotal

• Partial pressure of component gas i is given by Pi = i * P total where i = ni/ntotal

1.5 THE FARADAY AND THE MOLE

• Charge [Q] = current [I] * time [t]

Unit: C unit: A unit: s

• No. of moles of e- = no. of faradays = charge / 96500 C

1.7 CHEMICAL EQUATIONS AND STOCHIOMETRY

• Primary standard: a substance that can be used to prepare a standard solution without standardization (i.e. by dissolving a known mass of that substance in a solvent to give a known volume of the solution, the molarity of that solution can be calculated), e.g. (COOH)2.2H2O [acid], aminosulphonic acid (sulphamic acid) NH2SO3H [acid], anhydrous Na2CO3(s) [base], K2Cr2O7(s) [OA], KIO3(s) [OA], Na2C2O4(s) [RA], Fe(NH4)2(SO4)2.6H2O ammonium iron(II) sulphate(VI)-6-water [RA]

• Primary stand must be very pure / with constant composition does not absorb moisture or lose water easily, stable in air, soluble.

• Preparation of primary standard:• Standardization involving redox agents:

• Preparation of standard I2(aq):

• Titration of iodine (in flask) against thiosulphate (in burette)

• Standardization of other oxidizing agents:• Back titration

Common Errors in Section 1 particles are helium atoms, particles are

electrons particles are helium neclei, particles are fast

moving electrons

Mass spectrometer responds to the mass of particles It should be mass to charge ratio of particle, NOTmass

When using ideal gas equation PV = nRT, the unitsof P, V & T are respectively atm, dm3 & oCrespectively.

Unit of P is Nm-2 or Pa, unit of V is m3, unit of T isK

When using the equation P = (/M) RT ( isdensity, M is molar mass), the units of density & Mare g dm-3 & g respectively

Unit of and M must be consistent: density (), ing m-3 [or kg m-3]). Molar mass in g mol-1 [or kgmol-1], volume of V (in m3) at a pressure P (Nm-

2,Pa) During the electrolysis of CuSO4(aq) using copper

electrodes, the higher the concentration of CuSO4,the larger is the amount of copper deposited on thecathode

The no. of moles of Cu deposited is given by the no.of faradays (no. of moles of e-) passing through theelectrolytic cell, NOT by the molarity of theelectrolyte.

For the titration of iodine solution (in flask) withthiosulphate solution (in burette), the end point isdetected by the addition of starch to the iodinesolution before the titration.

Since the starch indicator will irreversibly combinewith iodine at high concentration, it is added onlywhen the iodine solution has just turned to paleyellow. The mixture then turns blue. The endpoint is reached when the blue color is decolorized.