Sodium hydrogencarbonate, NaHCO₃, decomposes on heating according to the equation: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g). A student heats a sample of NaHCO₃ until decomposition is complete. Explain how the student could use the masses recorded before and after heating to determine the percentage yield of Na₂CO₃, and state one assumption that must be made for this calculation to be valid.

IB DP Chemistry Standard Level (2023 syllabus) — R2.1 How much? The amount of chemical change · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Calculate the theoretical yield of Na₂CO₃ from the initial mass of NaHCO₃ using the stoichiometric ratio 2 mol NaHCO₃ → 1 mol Na₂CO₃.
2. Convert the initial mass of NaHCO₃ to moles, halve it to give the moles of Na₂CO₃ that should form, then multiply by the molar mass of Na₂CO₃ to give the theoretical mass.
3. Measure the mass of the solid residue after heating; this is the actual mass of Na₂CO₃ produced.
4. Calculate the percentage yield: (actual mass ÷ theoretical mass) × 100.
5. Assumption: the residue contains only Na₂CO₃ (no unreacted NaHCO₃ or other solids).

Examiner tips

  • Show the stoichiometric calculation step‑by‑step; use the 2:1 ratio. Include the formula for percentage yield. State the assumption explicitly. Use correct units (g, mol, %).

Common mistakes

  • Using the wrong mole ratio (e.g. 1:1 instead of 2:1). Calculating yield from the mass of gas rather than the solid residue. Failing to state the assumption that the residue is pure Na₂CO₃.

Mark scheme (4 marks)

  1. The theoretical yield of Na₂CO₃ is calculated from the initial mass of NaHCO₃ using stoichiometric mole ratios (2 mol NaHCO₃ produces 1 mol Na₂CO₃).
  2. The actual yield of Na₂CO₃ is obtained from the mass of solid remaining after heating (the residue mass).
  3. Percentage yield is calculated as (actual yield / theoretical yield) × 100.
  4. A valid assumption is that the residue consists entirely / only of Na₂CO₃ (i.e., no NaHCO₃ remains unreacted and no other solid product is formed).

Key terms in this question

percentage yield

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