O Level / IGCSE Chemistry chapter 3: Stoichiometry MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

70 exam-style questions on Stoichiometry, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

70 questions · 3 topics · 10 per test

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What chapter 3 covers

Ten Stoichiometry questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. Why is the relative atomic mass of chlorine, 35.5, not a whole number?

  1. A
    chlorine atoms contain half a neutron on average
  2. B
    the mass of the electrons is included
  3. C
    chlorine atoms lose mass when they form molecules
  4. D
    it is an average of the masses of its isotopes, weighted by how common each one is
Show answer

Answer: D. Relative atomic mass is the weighted average mass of the isotopes compared with 1/12 of the mass of a carbon-12 atom. No single chlorine atom has a mass of 35.5.

2. Which statement defines the molecular formula of a compound?

  1. A
    the number and type of each atom in one molecule
  2. B
    the simplest whole-number ratio of the atoms of each element
  3. C
    the arrangement of the bonds between atoms in one molecule
  4. D
    the total mass of all the atoms in one molecule
Show answer

Answer: A. The molecular formula gives the actual number of each type of atom in one molecule. The simplest ratio is the empirical formula, the bond arrangement is the structural formula and the mass is the Mr.

3. Solid calcium carbonate reacts with dilute hydrochloric acid. Which equation, including state symbols, is correct?

  1. A
    CaCO3(aq) + 2HCl(aq) → CaCl2(s) + H2O(l) + CO2(g)
  2. B
    CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
  3. C
    CaCO3(s) + HCl(aq) → CaCl(aq) + H2O(l) + CO2(g)
  4. D
    CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2(g) + CO2(g)
Show answer

Answer: B. Calcium carbonate is an insoluble solid and calcium chloride is soluble. Calcium ions are Ca2+, so two HCl are needed; a carbonate gives water and carbon dioxide, not hydrogen.

4. How many atoms are there in one formula unit of aluminium sulfate, Al2(SO4)3?

  1. A
    11
  2. B
    15
  3. C
    17
  4. D
    19
Show answer

Answer: C. Al: 2. The bracket multiplies everything inside by 3, giving 3 S and 3 × 4 = 12 O. Total = 2 + 3 + 12 = 17 atoms.

5. Aqueous sodium hydroxide is added to aqueous copper(II) sulfate and a blue precipitate forms. Which is the ionic equation, with state symbols?

  1. A
    Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
  2. B
    Cu2+(aq) + 2OH−(aq) → Cu(OH)2(aq)
  3. C
    Cu2+(aq) + OH−(aq) → CuOH(s)
  4. D
    Na+(aq) + SO42−(aq) → Na2SO4(aq)
Show answer

Answer: A. Only the ions that form the precipitate appear; Na+ and SO42− are spectator ions. Cu2+ needs two OH−, and a precipitate is a solid, (s).

6. What is the mass of 600 cm3 of methane, CH4, at room temperature and pressure?

  1. A
    0.025 g
  2. B
    0.40 g
  3. C
    4.0 g
  4. D
    400 g
Show answer

Answer: B. 600 cm3 = 0.600 dm3; 0.600 ÷ 24 = 0.025 mol; 0.025 × 16 = 0.40 g. 400 g comes from forgetting to convert cm3 to dm3, and 0.025 g stops at the number of moles.

7. Element Y has two isotopes, of mass numbers 63 and 65. The relative atomic mass of Y is 63.5. Which statement is correct?

  1. A
    the two isotopes are equally common
  2. B
    the isotope of mass 63 makes up 75% of the atoms
  3. C
    the isotope of mass 65 is the more common
  4. D
    some atoms of Y have a mass of 63.5
Show answer

Answer: B. 63.5 is much closer to 63 than to 65, so Y-63 is more common: 63x + 65(1 − x) = 63.5 gives x = 0.75. An average of 63.5 does not mean any single atom has that mass.

8. In which compound is the molecular formula the same as the empirical formula?

  1. A
    methane, CH4
  2. B
    glucose, C6H12O6
  3. C
    hydrogen peroxide, H2O2
  4. D
    ethene, C2H4
Show answer

Answer: A. CH4 cannot be simplified. The others simplify to CH2, HO and CH2O.

9. What is the formula of iron(III) hydroxide? (hydroxide ion OH−)

  1. A
    FeOH3
  2. B
    Fe(OH)2
  3. C
    Fe3OH
  4. D
    Fe(OH)3
Show answer

Answer: D. Iron(III) means Fe3+, so three OH− ions are needed. Brackets are needed: FeOH3 would mean one O and three H. Fe(OH)2 is iron(II) hydroxide.

10. Which is the correct ionic equation for the neutralisation of hydrochloric acid by sodium hydroxide?

  1. A
    H+ + Cl− → HCl
  2. B
    Na+ + Cl− → NaCl
  3. C
    HCl + NaOH → NaCl + H2O
  4. D
    H+ + OH− → H2O
Show answer

Answer: D. Na+ and Cl− are spectator ions. The reaction is between H+ and OH− to form water.

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Ways to practise this chapter

Stoichiometry: questions students ask

How many Stoichiometry MCQs are there?

70, each with the answer and a short explanation, across 3 topics: Formulae, Relative masses of atoms and molecules, The mole and the Avogadro constant. Every chapter test draws ten of them, so you can take it many times.

Is the O Level / IGCSE Chemistry chapter 3 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

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