AQA GCSE Chemistry
AQA GCSE Chemistry 4.3.2 Moles and Masses of Pure Substances Exam Questions
Free AQA GCSE Chemistry exam questions for 4.3.2 Moles and Masses of Pure Substances, with a mark scheme for every pack. Covers moles and the Avogadro constant, reacting masses, using moles to balance equations, and limiting reactants (Higher tier only).
4.3.2 Moles and Masses of Pure Substances exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA GCSE Chemistry 4.3.2 Moles and Masses of Pure Substances
Section 4.3.2 of AQA GCSE Chemistry introduces the mole, and almost all of it is Higher tier only. You learn that one mole of a substance contains 6.02 x 10^23 particles and has a mass in grams equal to its relative formula mass. You then calculate reacting masses from balanced equations, use masses to work out the balanced equation, and identify the limiting reactant. It is assessed on Paper 1 and is the foundation for yield, concentration and gas volume calculations later in 4.3. Questions are usually multi-step calculations where clear working earns method marks even if the final answer is wrong.
What you need to know
- The mole (Higher tier only) Chemical amounts are measured in moles (mol). One mole contains the Avogadro constant, 6.02 x 10^23, of atoms, molecules or ions. The mass of one mole in grams equals the Ar or Mr.
- Moles equation (Higher tier only) moles = mass (g) / Mr. Rearrange to find mass = moles x Mr.
- Reacting masses (Higher tier only) Use the balanced equation's mole ratio: convert the known mass to moles, use the ratio to find moles of the other substance, then convert back to mass.
- Balancing from masses (Higher tier only) Convert the masses of each reactant and product to moles, divide by the smallest number of moles to get a whole-number ratio, and use these as the balancing numbers.
- Limiting reactants (Higher tier only) The reactant that is completely used up limits the amount of product. The other reactant is in excess. Work out moles of each and compare with the ratio in the equation.
Common mistakes in 4.3.2 Moles and Masses of Pure Substances
- Using the mass ratio instead of the mole ratio. Convert to moles before using the numbers in the balanced equation.
- Using Ar instead of Mr for diatomic elements, for example 16 for oxygen gas instead of 32 for O2.
- Rounding too early in a multi-step calculation. Keep full figures until the final answer.
- Assuming the reactant with the smaller mass is the limiting reactant. Compare moles against the equation's ratio, not masses.
- Forgetting units: mass in g, amount in mol, Mr has no units.
Try three 4.3.2 Moles and Masses of Pure Substances exam-style questions
1. Calculate the number of moles in 22 g of carbon dioxide, CO2. (Ar: C = 12, O = 16) (2 marks)
Show the mark scheme answer
Mr = 12 + 32 = 44; moles = 22 / 44 = 0.5 mol.
2. Magnesium burns in oxygen: 2Mg + O2 → 2MgO. Calculate the mass of magnesium oxide formed from 6.0 g of magnesium. (Ar: Mg = 24, O = 16) (3 marks)
Show the mark scheme answer
Moles Mg = 6.0 / 24 = 0.25 mol; ratio 2:2 so 0.25 mol MgO; Mr MgO = 40, mass = 0.25 x 40 = 10 g.
3. 4.0 g of hydrogen reacts with 16.0 g of oxygen: 2H2 + O2 → 2H2O. Identify the limiting reactant. (Ar: H = 1, O = 16) (3 marks)
Show the mark scheme answer
Moles H2 = 4.0 / 2 = 2.0 mol; moles O2 = 16.0 / 32 = 0.50 mol; 2.0 mol H2 would need 1.0 mol O2 but only 0.50 mol is available, so oxygen is the limiting reactant.
4.3.2 Moles and Masses of Pure Substances FAQs
What does AQA GCSE Chemistry 4.3.2 cover?
It covers the mole, the Avogadro constant, reacting mass calculations, using moles to balance equations and limiting reactants. Most of this content is Higher tier only.
Which paper is 4.3.2 on?
It is in section 4.3 Quantitative chemistry, assessed on AQA GCSE Chemistry Paper 1.
What is the best method for reacting mass questions?
Use three steps: mass to moles using moles = mass / Mr, use the mole ratio from the balanced equation, then moles back to mass. Write each step on a separate line so you can pick up method marks.
