Using moles to balance equations (HT only) - AQA GCSE Chemistry Exam Questions

Practise 18 exam-style questions on Using moles to balance equations (HT only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

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Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Rearrange the equation moles = mass ÷ Mr to make Mr the subject.

Use your rearranged equation to calculate the Mr of a gas when 0.25 mol of the gas has a mass of 11 g.

Model answer

Mr = mass ÷ moles
Mr = 11 ÷ 0.25 = 44

Explanation

Multiply both sides by Mr and then divide both sides by moles. A quarter of a mole has a mass of 11 g, so a whole mole has a mass of 44 g, giving Mr = 44 (this could be carbon dioxide).

Mark scheme

1 mark
Mr = mass ÷ moles
correct answer with no working scores full marks
1 mark
Mr = 11 ÷ 0.25 = 44
ignore units

Question 2

3 marks

Hydrogen reacts with chlorine to form hydrogen chloride. In an experiment, 0.20 g of hydrogen (H₂) reacted with 7.1 g of chlorine (Cl₂) to form 7.3 g of hydrogen chloride (HCl).

Use these masses to write the balanced equation for the reaction.

Relative atomic masses (Ar): H = 1, Cl = 35.5

Model answer

H₂: 0.20 ÷ 2 = 0.10 mol
Cl₂: 7.1 ÷ 71 = 0.10 mol
HCl: 7.3 ÷ 36.5 = 0.20 mol
Ratio = 1 : 1 : 2
H₂ + Cl₂ → 2HCl

Explanation

Hydrogen and chlorine are diatomic, so their Mr values are 2 and 71. The mole ratio 0.10 : 0.10 : 0.20 simplifies to 1 : 1 : 2, giving the balancing numbers directly.

Mark scheme

1 mark
moles: H₂ = 0.20 ÷ 2 = 0.10; Cl₂ = 7.1 ÷ 71 = 0.10; HCl = 7.3 ÷ 36.5 = 0.20
all three needed
1 mark
ratio 1 : 1 : 2
allow ecf
1 mark
H₂ + Cl₂ → 2HCl
allow ecf from ratio; an equation with no working scores 1 mark

Question 3

3 marks

Phosphorus (P₄) burns in oxygen to form phosphorus oxide (P₄O₁₀). In an experiment, 12.4 g of phosphorus reacted with 16.0 g of oxygen (O₂) to form 28.4 g of phosphorus oxide.

Use these masses to write the balanced equation for the reaction.

Relative formula masses (Mr): P₄ = 124, O₂ = 32, P₄O₁₀ = 284

Model answer

P₄: 12.4 ÷ 124 = 0.10 mol
O₂: 16.0 ÷ 32 = 0.50 mol
P₄O₁₀: 28.4 ÷ 284 = 0.10 mol
Divide by 0.10: 1 : 5 : 1
P₄ + 5O₂ → P₄O₁₀

Explanation

Even with unfamiliar substances, the method is the same: mass ÷ Mr for each substance, then simplify the ratio. Check: 4 P and 10 O atoms on each side.

Mark scheme

1 mark
moles: P₄ = 12.4 ÷ 124 = 0.10; O₂ = 16.0 ÷ 32 = 0.50; P₄O₁₀ = 28.4 ÷ 284 = 0.10
all three needed
1 mark
ratio 1 : 5 : 1
allow ecf
1 mark
P₄ + 5O₂ → P₄O₁₀
allow ecf from ratio

Question 4

4 marks

A student heated 0.48 g of magnesium in a crucible. The mass of magnesium oxide formed was 0.75 g.

The formula of magnesium oxide is MgO, so the expected ratio of magnesium atoms to oxygen atoms is 1 : 1.

Calculate the ratio of moles of magnesium atoms to moles of oxygen atoms in the student's product. Suggest one reason why the ratio is not 1 : 1.

Relative atomic masses (Ar): O = 16, Mg = 24

Model answer

Mass of oxygen = 0.75 - 0.48 = 0.27 g
Moles of Mg = 0.48 ÷ 24 = 0.020 mol
Moles of O = 0.27 ÷ 16 = 0.0169 mol
Ratio Mg : O = 0.020 : 0.0169 = 1 : 0.84
The ratio has too little oxygen, probably because not all the magnesium reacted, or some magnesium oxide escaped as smoke.

Explanation

Real experimental data rarely give perfect whole numbers. Here the product has less oxygen than expected (0.75 g instead of 0.80 g), so either some magnesium did not react or some product was lost. Recognising and explaining errors like this is an important skill.

Mark scheme

1 mark
mass of oxygen = 0.75 - 0.48 = 0.27 g
correct answer with no working scores full marks
1 mark
moles of Mg = 0.48 ÷ 24 = 0.020 and moles of O = 0.27 ÷ 16 = 0.0169
allow 0.017; allow ecf
1 mark
ratio Mg : O = 1 : 0.84
allow 1 : 0.8 to 1 : 0.85; allow 1.19 : 1; allow ecf
1 mark
not all the magnesium reacted / some magnesium oxide escaped (as smoke) when the lid was lifted
allow not heated to constant mass

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