This chapter discusses the mole concept, including defining the mole, deriving empirical and molecular formulas, stating Avogadro's Law, and applying the mole concept to ionic and molecular equations. It introduces the mole as the amount of substance containing 6x1023 particles. It provides examples of how to determine the empirical formula, molecular formula, and formula of a compound from composition data. It also discusses molar volume of gases and limiting reactants. Worked examples are included for many of these concepts.

1. Chapter 5
Mole Concept
LEARNING OUTCOMES
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Define the mole
Derive empirical and molecular formulae
State Avogadro’s Law
Apply the mole concept to ionic and molecular
equations

2. Chapter 5
Mole Concept
Introducing the Mole
A mole is the number of atoms or molecules in 1 g
of hydrogen or 12 g of carbon.
The number 6 x 1023 is called one mole or
Avogadro’s constant in honour of Amedeo
Avogadro.
1 mole of atoms of any element will have a
mass equal to its relative atomic mass,
expressed in grams.

3. Chapter 5
Mole Concept
Formulas
1. Mass of 1 mole of atoms = Ar in grams
Mass of the element in grams
2. Number of moles of atoms = Relative atomic mass, Ar
3. Mass of 1 mole of molecules = Mr in grams
Mass of the substance in grams
4. Number of moles of molecules = Relative molecular mass, Mr
5. Mass of substance containing 1 mole of particles = Molar mass
Actual mass of product obtained
6. Percentage yield = Theoretical mass of product obtainable

4. Chapter 5
Mole Concept
Empirical Formula
The empirical formula is the simplest formula.
 It shows the simplest ratio of the elements present in a
compound.
 Examples of empirical formulae are:
H2O, CO2, H2SO4, CH2, CH3


The following are not empirical formulae:
(a) C2H4, (b) C2H6, (c) C2H4O2
because they can be reduced to:
(a) CH2, (b) CH3, (c) CH2O

5. Chapter 5
Mole Concept
Molecular Formula
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The molecular formula is the true formula.
It shows all the atoms present in the molecule.
Examples of molecular formulae are:
H2O, H2O2, CO2, H2SO4, Cu(NO3)2
Note that H2O is water, and the molecular formula is the
same as the empirical formula.
H2O2 is hydrogen peroxide. Its empirical formula is HO.

6. Chapter 5
Mole Concept
Finding the Molecular Formula
Worked example 1
Propene has the empirical formula CH2. The relative
molecular mass of propene is 42. Find the molecular formula
of propene.
Solution
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
Let the molecular formula of propene be (CH2)n.
Since the Mr is 42, (12 + 1x2)n = 42
14n = 42
n = 42 = 3
14
Hence the molecular formula is 3x(CH2) = C3H6

7. Chapter 5
Mole Concept
Finding the Molecular Formula
Worked example 2
A hydrocarbon consists of 85.7% of carbon and 14.3% of hydrogen by mass.
(a) Find the empirical formula of the compound.
(b) If the molecular mass is 56, find the molecular formula.
Solution
C : H
(a)
85.7 : 14.3
85.7 : 14.3
12
1
7.14 : 14.3
7.14 7.14
1 : 2
The empirical formula is CH2.
(b) Let the molecular formula be (CH2)n.
Since the Mr is 56, (12 + 1x2)n = 56
14n = 56
n = 56 = 4
14
Hence the molecular formula is 4 x (CH2) = C4H8

8. Chapter 5
Mole Concept
Formula of a compound
A pure compound has a fixed chemical composition;
hence it can be represented by a chemical formula.
 For example, a molecule of water is made up of 2
atoms of hydrogen and 1 atom of oxygen, and its
molecular formula is H2O.
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
We can find the formula of a compound from its
percentage composition.

9. Chapter 5
Mole Concept
Finding the formula of a compound
Worked example 1
A compound of sodium contains the following percentage composition by mass:
32.4% of sodium, 22.6% of sulphur and 45.0% of oxygen. Find the formula of the
compound.
Solution
Na : S : O
Step 1: Write down the percentage:
32.4 : 22.6 : 45.0
Step 2: Divide each by the Ar:
32.4 : 22.6 : 45.0
(to get number of moles)
23
32
16
1.41 : 0.706 : 2.81
Step 3: Divide by the smallest number: 1.41 : 0.706 : 2.81
0.706 0.706 0.706
2
: 1 : 4
Step 4 : Write down the formula: Na2SO4

10. Chapter 5
Mole Concept
Finding the formula of a compound
Worked example 2
A compound contains 1.2 g of carbon, 3.2 g of oxygen and 0.2g of
hydrogen. Find the formula of the compound.
Solution
Step 1: Write down the mass ratio:
Step 2: Divide each mass by the Ar:
Step 3: Divide by the smallest number:
Step 4 : Write down the formula:
C
1.2
1.2
12
0.1
0.1
0.1
1
: O : H
: 3.2 : 0.2
: 3.2 : 0.2
16
1
: 0.2 : 0.2
: 0.2 : 0.2
0.1 0.1
: 2 : 2
CO 2H2 (or HCOOH)

11. Chapter 5
Mole Concept
Finding the formula of a compound
Worked example 3
A compound contains 48.6% of carbon, 43.2% of oxygen, with
the remainder being hydrogen. Find the formula of the compound.
Solution
Step 1: Find the % of hydrogen:
100 – 48.6 – 43.2 = 8.2 %
Step 2: Write down the % ratio:
Step 3: Divide each mass by the Ar:
Step 4: Divide by the smallest number:
Step 5: Multiply each number by 2:
Step 6 : Write down the formula:
C : O : H
48.6 : 43.2 : 8.2
48.6 : 43.2 : 8.2
12
16
1
4.05 : 2.7 : 8.2
4.05 : 2.7 : 8.2
2.7
2.7 2.7
1.5 : 1 : 3
3
: 2 : 6
C3O2H6 (or C2H5COOH)

12. Chapter 5
Mole Concept
Molar Volume of Gases
Avogadro’s Law states that equal volume of gases under
the same temperature and pressure contain the same
number of molecules.
Volume of 1 mole of gas = 24 dm3
Volume of gas = Number of moles x 24 dm 3
Number of moles = Volume of gas in dm3
24 dm3

13. Chapter 5
Mole Concept
Limiting Reactants
2H2(g) + O2(g)
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
2H2O(g)
2 moles of hydrogen gas react with one or more moles of
oxygen to form 2 moles of steam or water vapour.
Therefore we say that oxygen is in excess and hydrogen is
called the limiting reactant because the reaction stops when
hydrogen is used up.

14. Chapter 5
Mole Concept
Quick check 1
Find the formula of each of the following:
1.
A compound containing 75% carbon and 25% hydrogen by mass.
2.
A compound containing 46.7% silicon and 53.3% oxygen by mass.
3.
A compound consisting of 43.4% sodium, 11.3% carbon and 45.3%
oxygen by mass.
4.
A compound consisting of 2.8 g of iron combined with 1.2 g of
oxygen.
5.
(a) A compound containing 18.9% lithium,
64.9% oxygen and the rest carbon by mass.
(b) Give the name of this compound.
Solution

15. Chapter 5
Mole Concept
Quick check 2
1.
A hydrocarbon consists of 80% carbon and 20% hydrogen by
mass.
(a) Find the empirical formula of the compound.
(b) If the molecular mass is 30, find the molecular formula.
2.
An acid contains 40% carbon, 6.67% hydrogen and 53.33%
oxygen by mass.
(a) Find the empirical formula of the acid.
(b) If the molecular mass of the acid is 60, what is its molecular
formula?
3.
A compound called borazine has the following percentage
composition by mass: 40.74% boron, 51.85% nitrogen and the
rest hydrogen. Find the molecular formula of borazine, given it
has a relative molecular mass of 81.
Solution

16. Chapter 5
Mole Concept
Solution to Quick check 1
1.
CH4
2.
SiO2
3.
Na2CO3
4.
Fe2O3
5.
(a) Li2CO3
(b) lithium carbonate
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17. Chapter 5
Mole Concept
Solution to Quick check 2
1.
(a) Empirical formula: CH3
(b) Molecular formula: C2H6
2.
(a) Empirical formula: CH2O
(b) Molecular formula: C2H4O2 (CH3COOH)
3.
Molecular formula: B3N3H6
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