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Chemistry · Topic

Equations, moles and reacting masses

Balance first, then use moles. An unbalanced equation corrupts every later step.

2 original questions in the bank, worth 5 marks · above the two-per-topic floor

Drill this topic All equations, moles and reacting masses questions

The questions, with the examiner reasoning

Calculate3 marksBGCSE

Zinc reacts with hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. 6.5 g of zinc reacts completely. The relative atomic mass of zinc is 65 and the molar volume of a gas at room temperature is 24 dm³. Calculate the volume of hydrogen produced.

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Answer: 2.4 dm³

  1. Moles of zinc = 6.5 ÷ 65 = 0.10 mol
  2. The equation shows 1 mol Zn gives 1 mol H₂, so 0.10 mol H₂
  3. Volume = 0.10 × 24 = 2.4 dm³

Examiner technique: Balance, moles, ratio, then convert. Four short lines that earn marks even if one number goes wrong — the ratio line alone is usually worth one.

Balance2 marksBoth boards

Balance the equation __Fe + __O₂ → __Fe₂O₃ and state whether the reaction is oxidation or reduction.

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Answer: 4Fe + 3O₂ → 2Fe₂O₃; oxidation (iron gains oxygen)

  1. Oxygen atoms need to match: 3 O₂ gives 6 oxygen atoms, so 2Fe₂O₃
  2. Iron then needs 4 atoms on the left
  3. Gaining oxygen is oxidation

Examiner technique: Balance the odd element last. Starting with iron leaves you adjusting it twice, which is where the second mistake usually happens.