Ammonia is produced in the Haber process. The equation for the reaction is: N₂ + 3H₂ → 2NH₃. A student wants to produce ammonia in the laboratory using 5.6 g of nitrogen gas. The relative atomic masses are: N = 14, H = 1. Explain how the student would calculate the theoretical yield of ammonia from this mass of nitrogen, and state the theoretical yield in grams.

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).

The Haber process produces ammonia from nitrogen and hydrogen. In industry, the reaction conditions are 450°C, 200 atm and an iron catalyst. In a laboratory experiment, a student uses 5.6 g of pure nitrogen gas with excess hydrogen gas.

Model answer (5 marks)

Calculate the moles of nitrogen:
0.200 mol N₂ = 5.6 g ÷ 28 g mol⁻¹
Use the stoichiometry of the balanced equation:
1 mol N₂ → 2 mol NH₃
0.200 mol N₂ → 0.400 mol NH₃
Find the molar mass of ammonia:
Mr NH₃ = 14 + 3×1 = 17 g mol⁻¹
Calculate the mass of NH₃ produced:
0.400 mol × 17 g mol⁻¹ = 6.8 g
Therefore the theoretical yield of ammonia is 6.8 g.

Examiner tips

  • Show the step‑by‑step calculation of moles, use the correct stoichiometric ratio, calculate the molar mass of NH₃, and multiply moles by molar mass to give the yield in grams.
  • Include the unit ‘g’ in the final answer and state the yield clearly.

Mark scheme (5 marks)

  1. Calculate the number of moles of nitrogen: moles = mass ÷ Mr = 5.6 ÷ 28 = 0.2 mol
  2. Use the molar ratio from the equation: 1 mol N₂ produces 2 mol NH₃, so 0.2 mol N₂ produces 0.4 mol NH₃
  3. Calculate the Mr of ammonia (NH₃): Mr = 14 + (3 × 1) = 17
  4. Calculate theoretical yield using mass = moles × Mr: mass = 0.4 × 17
  5. Correct theoretical yield stated as 6.8 g

Key terms in this question

theoretical yield

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