⚖️ Mole Concept & Stoichiometry

Secondary School Chemistry • MalawiHub

1. Introduction

The mole is a convenient unit for counting very large numbers of particles. Stoichiometry is the calculation of the quantities of substances involved in chemical reactions.

Learning Objectives

Define the mole and Avogadro's constant.
Calculate relative formula mass.
Calculate number of moles from mass.
Use chemical equations to calculate reacting quantities.
Calculate concentration and gas volumes.
Determine empirical and molecular formulae.

2. The Mole

One mole contains approximately 6.02 × 10²³ particles.

This number is called Avogadro's constant.

n = m / M

where n = number of moles, m = mass in grams and M = molar mass in g mol⁻¹.

3. Relative Formula Mass

Relative formula mass is found by adding the relative atomic masses of all atoms in the formula.

Example: Calculate the relative formula mass of H₂SO₄.

H = 1, S = 32, O = 16
Mᵣ = (2 × 1) + 32 + (4 × 16)
Mᵣ = 98

4. Calculating Moles from Mass

Example: Calculate the number of moles in 18 g of water, H₂O.

Mᵣ of H₂O = 18
n = m/M
n = 18/18
n = 1 mol

5. Mole Ratios from Equations

The coefficients in a balanced equation give the mole ratio of the reacting substances.

N₂ + 3H₂ → 2NH₃

The ratio is:
1 mol N₂ : 3 mol H₂ : 2 mol NH₃

6. Mass–Mass Calculations

Use the balanced equation to convert from the known substance to the required substance.

Example:
CaCO₃ → CaO + CO₂

100 g of CaCO₃ represents 1 mol because Mᵣ(CaCO₃) = 100.
Therefore 100 g CaCO₃ produces 1 mol CaO.
Mᵣ(CaO) = 56.
So the mass of CaO produced is 56 g.

7. Concentration

c = n/V

Concentration in mol dm⁻³ is calculated by dividing moles by volume in dm³.

Example: 0.50 mol of NaOH is dissolved to make 2.0 dm³ of solution.
c = 0.50/2.0
c = 0.25 mol dm⁻³

8. Gas Volumes

At room temperature and pressure, one mole of many gases occupies approximately 24 dm³.

n = V/24

Example: What volume is occupied by 2 mol of gas at room temperature and pressure?
V = 2 × 24
V = 48 dm³

9. Empirical Formula

The empirical formula shows the simplest whole-number ratio of atoms in a compound.

Method:

Convert each mass or percentage to moles.
Divide all mole values by the smallest value.
Convert to the simplest whole-number ratio.
Write the formula.

10. MANEB Examination Focus

Use correct units.
Write the balanced equation first.
Calculate relative formula mass carefully.
Use mole ratios from the balanced equation.
Convert cm³ to dm³ when necessary.
Show working clearly.

MANEB-style Question

Calculate the number of moles in 5.85 g of NaCl. (Na = 23, Cl = 35.5)

Mᵣ(NaCl) = 23 + 35.5 = 58.5
n = m/M
n = 5.85/58.5
n = 0.10 mol

11. Practice Exercises

Calculate Mᵣ of CaCO₃. (Ca=40, C=12, O=16)
Calculate the moles in 9 g of water.
Calculate the concentration of 0.20 mol in 500 cm³ of solution.
What volume does 0.50 mol of gas occupy at RTP?
Explain why equations must be balanced before stoichiometric calculations.

Answers

100
0.50 mol
0.40 mol dm⁻³
12 dm³
Because the coefficients provide the correct mole ratio between reactants and products.

12. Common Mistakes

Forgetting to balance the equation.
Using mass instead of moles in a mole ratio.
Forgetting to convert cm³ to dm³.
Giving answers without units.
Using the wrong relative atomic mass.

13. Exam Tip

For calculation questions, use this order: balanced equation → known moles → mole ratio → required moles → final quantity.