A student reacts 24 g of magnesium with excess hydrochloric acid. The equation for the reaction is: Mg + 2HCl → MgCl₂ + H₂. Explain, using the concept of moles, why the amount of hydrogen gas produced is the same number of moles as the magnesium used. Then state what mass of hydrogen gas would be produced. (Relative atomic masses: Mg = 24, H = 1)

AQA A-Level Chemistry (7405) — 3.1.2 Amount of substance · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Moles of Mg = 24 g ÷ 24 g mol⁻¹ = 1 mol
The balanced equation shows a 1:1 mole ratio of Mg to H₂
Therefore moles of H₂ produced = 1 mol
Relative molecular mass of H₂ = 2 g mol⁻¹
Mass of H₂ = 1 mol × 2 g mol⁻¹ = 2 g

Examiner tips

  • Show the calculation for moles of Mg first
  • State the mole ratio from the balanced equation
  • Use the ratio to find moles of H₂
  • Convert moles of H₂ to mass with the molecular mass

Common mistakes

  • Using the wrong mole ratio (e.g. 2:1)
  • Confusing atomic mass of H with molecular mass of H₂
  • Failing to show the calculation for moles of Mg

Mark scheme (5 marks)

  1. Moles of Mg = mass ÷ relative atomic mass = 24 ÷ 24 = 1 mol
  2. The mole ratio of Mg to H₂ in the balanced equation is 1:1
  3. Therefore moles of H₂ produced = 1 mol
  4. Relative molecular mass of H₂ = 2
  5. Mass of H₂ = 1 × 2 = 2 g

Key terms in this question

moles · relative atomic mass

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