GCSE & IGCSE chemistry practice

Moles questions and answers

A mole is an amount of substance: it contains $6.02 \times 10^{23}$ particles (the Avogadro constant). The mass of one mole is the $M_r$ in grams, so moles = mass ÷ $M_r$ . Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, S = 32, Cl = 35.5, K = 39, Ca = 40, Fe = 56, Cu = 63.5.

8 questions · Higher tier · Every answer is checked as you type it, with a full worked solution. Updated 27 Oct 2026.

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Where moles comes up

GCSE (AQA)	Chemistry and Combined Science, Higher only: the mole, the Avogadro constant (6.02 × 10²³) and number of moles = mass ÷ Mr.
Cambridge IGCSE (0620)	Supplement (Extended): the mole and the Avogadro constant; use amount (mol) = mass (g) ÷ molar mass (g/mol) to find amount, mass, molar mass or number of particles.

The method

Moles = mass (g) ÷ Mr.
Mass = moles × Mr; Mr = mass ÷ moles.
Number of particles = moles × 6.02 × 10²³.
Work out the Mr first if you need it.

Moles questions

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Warm-up

Question 1Warm-upHigher

How many moles are in 36 g of water ( $M_r = 18$ )?

mol

Question 2Warm-upHigher

How many moles are in 11 g of carbon dioxide? (C = 12, O = 16)

mol

Exam-style

Question 3Exam-styleHigher

What is the mass of 3 moles of sodium chloride? (Na = 23, Cl = 35.5)

Question 4Exam-styleHigher

0.5 moles of a substance has a mass of 20 g. What is its $M_r$ ?

Question 5Exam-styleHigher

How many moles are in 4 g of magnesium oxide, MgO? (Mg = 24, O = 16)

mol

Question 6Exam-styleHigher

How many molecules are in 2 moles of oxygen gas? (Avogadro constant = $6.02 \times 10^{23}$ )

molecules

Challenge

Question 7ChallengeHigher

What is the mass of 0.2 moles of oxygen gas, $\text{O}_2$ ? (O = 16)

Question 8ChallengeHigher

How many atoms are in 6 g of carbon? (C = 12, Avogadro constant = $6.02 \times 10^{23}$ )

atoms

Common mistakes

Dividing Mr by mass.
Using kg instead of g.
Forgetting that a diatomic gas like O₂ has Mr 32, not 16.

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Sources

The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:

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