Chapter 3b Chemical Formulae

8/8/2019 Chapter 3b Chemical Formulae

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ChemicalChemicalFormulaeFormulae


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Chemical formula is a representation

of a chemical substance.

NH3Each ammonia molecule consists of 1

nitrogen atom & 3 hydrogen atoms.

Number of

hydrogen atoms

Types of atoms


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Based on chemical formula, we

know

the composition of a substance

the elements that make up thesubstance.

the ratio ornumber of atoms of

each element in the substance.


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Compound Chemical

Formula

Elements

present

Ratio of numbers

of atom

Carbon

dioxide

CO2

Water H2O

Ammonia NH3

Sulphuric

acidH2SO4

Magnesium

hydroxide

Mg(OH)2


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Molecular FormulaMolecular Formula

Shows the actual number of

atoms of each element present in

one molecule of the compound.


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Empirical FormulaEmpirical Formula

Gives the simplest whole number

ratio of atoms of each elements

present in the compound.


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Example :

Hexene :

Molecular formula : C6H12.

Empirical formula : CH2.


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Example :

Dinitrogen tetraoxide :

Molecular formula : N2O4.

Empirical formula : NO2.


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Example :

Glucose :

Molecular formula : C6H12O6.

Empirical formula : ?


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Example :

Ethanoic acid :

Molecular formula : CH3COOH.



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Example :

Vitamin C :

Molecular formula : C6H8O6.



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Example :

Carbon dioxide :

Molecular formula : CO2.



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Empirical Formula Molecular Formula

Similarities

Both show the elements present in the

compound.

Both show the ratio ofatoms of each

elementin the compound.


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Empirical Formula Molecular Formula

Differences

Shows the simplest

ratio ofatoms of

each elements in

a compound.

Shows the actual

ratio ofatoms of

each elements in

a molecule.


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Determining Empirical FormulaeDetermining Empirical Formulae

By experiments.

Steps :

1. Find the mass of each element

2. Convert to moles of atoms

3. Find the simplest ratio of moles


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Example 1

1.08 g of aluminium combines chemically

with 0.96 g oxygen to form an oxide.

What is the empirical formula of theoxide ?

[ Relative atomic mass: O, 16 ; Al, 27 ]


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Example 2

1.69 g of iron combines chemically

with 0.72 g of oxygen.

Find the empirical formula of the oxide.[ Relative atomic mass: O, 16 ; Fe, 56 ]


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Example 3

0.91 g of aluminium burns in air to

form 1.7 g of aluminium oxide.

Find the empirical formula of theoxide.

[ Relative atomic mass: O, 16 ; Al, 27 ]


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Example 7

The decomposition of 7.36 g of acompound produces 6.93 g of oxygen.The rest of the mass is hydrogen.

If the relative molecular mass of thiscompound is 34.0 g, calculate itsmolecular formula.

[ Relative atomic mass : H, 1 ; O, 16 ]


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Determining empirical formula of

Magnesium oxide

A crucible & its lid are weighed.

A magnesium ribbon is cleanedwith sand paper remove the oxide layer.

The ribbon is coiled & is placed

in the crucible. The content isweighed.


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The crucible is heated

strongly without the lid.

When magnesium starts

to burn, the crucible is

covered with its lid.

Using a pair of tongs, the

lid is raised a little at

intervals allow oxygen

to enter the crucible &react with Mg.

The crucible is then quickly

covered with its lid to

prevent the white fumesfrom escaping. This would

affect the accuracy of the

mass obtained.

When burning is complete,

the lid is removed & the

crucible is heated strongly

for 1 to 2 min.


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The crucible is

allowed to cool toroom temperature

with its lid on.

The crucible & itscontent is weighed

again.

The process of

heating, cooling &weighing are

repeated until a

constant mass is

obtained ensurethat the reaction is

complete.


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Example 4

Phosphoric acid has the percentage

composition as follows :

H, 3.06% ; P, 31.63% ; O, 65.31%What is the empirical formula of the acid?

[Relative atomic mass: H, 1; O, 16; P, 31]


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Example 5

The percentage composition ofcalcium carbonate :

Ca, 40% ; C, 12% ; O, 48%

What is the empirical formula ofcalcium carbonate ?

[Relative atomic mass: Ca, 40 ; C, 12;

O, 16]


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Example 6

The empirical formula of ethene is(CH3)n. Its molecular mass is 30.

Calculate the formula of this

compound.


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Determining empirical formula of

copper (II) oxide

Zinc

Hydrochloric

acid

Combustion tube

(brown)


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Mass of combustion tube with porcelain

dish is weighed. A spatulaful of copper (II) oxide is added

on the porcelain dish. Weigh again.

Hydrogen gas is allowed to flow into thecombustion tube to remove air in the tube.

Function ofanhydrous calcium chloride is

to dry the hydrogen gas.


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Air in the combustion tube must be totally

removed before heating. A mixture ofhydrogen gas & oxygen gas will cause an

explosion when lighted.

To determine whether all the air has beenremoved, gas that comes out from small

hole is collected in a test tube. Gas is

tested with a lighted wooden splinter. Gas

burns without pop shows air has beentotally removed.


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Excess hydrogen gas flows out from small

hole is burnt to prevent air from enteringthe tube again.

Copper (II) oxide (black) is heated strongly

until it turns completely brown (copper). Flame is turn off but the flow of hydrogen

is continued until it is cool or the hot

copper produced will react with oxygen

from the air to form copper (II) oxide again.


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Mass of combustion tube with copper is

weighed again. The process ofheating, cooling &

weighing are repeated until a constant

mass is obtained. This is to ensure that all

copper (II) oxide has changed into copper.

This method applicable for oxides oflow

reactivity metals : tin (II) oxide, lead (II)

oxide.


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Mg + copper (II) oxide ?

Cannot. Both the reactants & products aresolids & thus the mass of copper & oxygen

cannot be determined.

Mg(s) + CuO(s) MgO(s) + Cu(s)


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Ionic Formulae

Made up ofpositively-charged ions (cations)& negatively-charged ions (anions).

Zinc bromide

Zn2+ Br

ZnBr2


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Cation Formula Anion Formula

Sodium ion Na+ Chloride ion Cl

Potassium ion K+ Bromide ion Br

Zinc ion Zn2+ Iodide ion I

Magnesium ion Mg2+ Oxide ion O2

Calcium ion Ca2+ Hydroxide ion OH

Iron (II) ion Fe2+ Sulphate ion SO42

Iron (III) ion Fe3+ Carbonate ion CO3

2

Copper (II) ion Cu2+ Nitrate ion NO3

Ammonium ion NH 4+ Phosphate ion PO4

3


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Construct the chemical formula :

Sodium chloride

Iron (II) iodide

Potassium sulphate

Ammonium hydroxide

Copper (II) phosphate

Zinc carbonate

Magnesium nitrate


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Name the ionic compounds :

AgCl

Ba(NO3)2

CaSO4

K2CO3

(NH4)3PO4


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Homework :

Text book

Activity 3.9 (P.46)

Activity 3.10 (P.47)


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Chapter 3b Chemical Formulae