sciencegcse.co.uk← All lessons in this topic
Chemistry Paper 1 / Quantitative chemistry
Lesson 3.10 · AQA Combined Science

Higher Tier: reacting masses

Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.

The equation gives a mole ratio

Higher Tier only. The coefficients in a balanced equation give the mole ratio.
2Mg + O₂ → 2MgO means 2 mol Mg reacts with 1 mol O₂ to form 2 mol MgO. It does not mean 2 g + 1 g → 2 g.

Use three steps every time

1. Convert the known mass to moles.
2. Use the balanced equation ratio to find moles of the other substance.
3. Convert those moles to mass.

Worked example: magnesium oxide

Find the maximum MgO mass from 12 g Mg with oxygen in excess. Ar: Mg 24, O 16.
1. Mg amount = 12 ÷ 24 = 0.50 mol.
2. Mg:MgO = 2:2 = 1:1, so MgO = 0.50 mol.
3. Mr(MgO) = 40; mass = 0.50 × 40 = 20 g.

When the ratio is not 1:1

N₂ + 3H₂ → 2NH₃.
For 28 g N₂ and excess H₂:
N₂ = 28 ÷ 28 = 1 mol.
NH₃ = 1 × 2 = 2 mol.
Mass NH₃ = 2 × 17 = 34 g.
The calculation assumes the reaction completes as shown and the other reactant is available in excess.

Remember

Convert mass to moles, use the coefficient ratio, then convert moles back to mass.