Copper(II) sulfate solution reacts with excess zinc powder. The equation for the reaction is: CuSO₄ + Zn → ZnSO₄ + Cu. A student wants to obtain 3.2 g of copper from this reaction. Calculate the minimum mass of zinc that must be used. Show your working. (Relative atomic masses: Cu = 64, Zn = 65, S = 32, O = 16)

Edexcel A-Level Chemistry (9CH0) — 5.1 Calculations using moles · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Molar mass of Cu = 64 g mol⁻¹

Moles of Cu required = 3.2 g ÷ 64 g mol⁻¹ = 0.05 mol

From the balanced equation CuSO₄ + Zn → ZnSO₄ + Cu the molar ratio Zn:Cu = 1:1

Moles of Zn needed = 0.05 mol

Molar mass of Zn = 65 g mol⁻¹

Mass of Zn = 0.05 mol × 65 g mol⁻¹ = 3.25 g

Therefore a minimum of 3.25 g of zinc is required.

Examiner tips

  • Show each calculation step and units; state the molar masses used; use the 1:1 ratio explicitly; round to two significant figures as given in the data.

Common mistakes

  • Using the wrong molar mass for Cu or Zn; forgetting to convert grams to moles; misapplying the stoichiometric ratio; rounding to an incorrect number of significant figures.

Mark scheme (5 marks)

  1. Correct molar mass of copper stated or used: Mr(Cu) = 64 g/mol
  2. Correct calculation of moles of copper: moles = 3.2 ÷ 64 = 0.05 mol
  3. Use of 1:1 molar ratio from the equation to state that moles of zinc = moles of copper = 0.05 mol
  4. Correct molar mass of zinc stated or used: Mr(Zn) = 65 g/mol
  5. Correct final answer: mass of zinc = 0.05 × 65 = 3.25 g

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