1. Mole Concept
Chapter 5
LEARNING OUTCOMES
Define the mole
Derive empirical and molecular formulae
State Avogadro’s Law
Apply the mole concept to ionic and
molecular equations
2. 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.
Introducing the Mole
Chapter 5
Mole Concept
3. Chapter 5
Formulas
1. Mass of 1 mole of atoms = Ar in grams
2. Number of moles of atoms =
3. Mass of 1 mole of molecules = Mr in grams
4. Number of moles of molecules =
5. Mass of substance containing 1 mole of particles = Molar mass
6. Percentage yield =
Mass of the element in grams
Relative atomic mass, Ar
Mass of the substance in grams
Relative molecular mass, Mr
Actual mass of product obtained
Theoretical mass of product obtainable
Mole Concept
4. 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
Chapter 5
Empirical Formula
Mole Concept
5. 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.
Chapter 5
Molecular Formula
Mole Concept
6. 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
Chapter 5
Finding the Molecular Formula
Propene has the empirical formula CH2. The relative
molecular mass of propene is 42. Find the molecular formula
of propene.
Worked example 1
Solution
Mole Concept
7. 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
Chapter 5
Finding the Molecular Formula
Worked example 1
Solution
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.
Mole Concept
8. 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.
We can find the formula of a compound from its
percentage composition.
Chapter 5
Mole Concept
9. Step 4 : Write down the formula: Na2SO4
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.
Worked example 1
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
Chapter 5
Finding the formula of a compound
Mole Concept
10. Step 4 : Write down the formula: CO2H2 (or HCOOH)
A compound contains 1.2 g of carbon, 3.2 g of oxygen and 0.2g of
hydrogen. Find the formula of the compound.
C : O : H
Step 1: Write down the mass ratio: 1.2 : 3.2 : 0.2
Step 2: Divide each mass by the Ar: 1.2 : 3.2 : 0.2
12 16 1
0.1 : 0.2 : 0.2
Step 3: Divide by the smallest number: 0.1 : 0.2 : 0.2
0.1 0.1 0.1
1 : 2 : 2
Chapter 5
Finding the formula of a compound
Worked example 2
Solution
Mole Concept
11. Step 1: Find the % of hydrogen: 100 – 48.6 – 43.2 = 8.2 %
Step 2: Write down the % ratio: C : O : H
48.6 : 43.2 : 8.2
Step 3: Divide each mass by the Ar: 48.6 : 43.2 : 8.2
12 16 1
4.05 : 2.7 : 8.2
Step 4: Divide by the smallest number: 4.05 : 2.7 : 8.2
2.7 2.7 2.7
1.5 : 1 : 3
Step 5: Multiply each number by 2: 3 : 2 : 6
Step 6 : Write down the formula: C3O2H6 (or C2H5COOH)
A compound contains 48.6% of carbon, 43.2% of oxygen, with
the remainder being hydrogen. Find the formula of the compound.
Chapter 5
Finding the formula of a compound
Worked example 3
Solution
Mole Concept
12. Chapter 5
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 dm3
Number of moles = Volume of gas in dm3
24 dm3
Mole Concept
13. Limiting Reactants
2H2(g) + O2(g) 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.
Chapter 5
Mole Concept
14. 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
Chapter 5
Quick check 1
Mole Concept
15. 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
Chapter 5
Quick check 2
Mole Concept
