A student carries out a reaction to produce calcium carbonate as a precipitate. The theoretical yield is 25.0 g, but the student only collects 18.5 g of dry calcium carbonate. Explain why the actual yield is less than the theoretical yield, and describe TWO reasons that could account for this difference in this type of reaction.

Edexcel GCSE Chemistry (1CH0) — 5.2 Quantitative analysis (Chem only) · Explain · 4 marks · View as Markdown

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

Calcium carbonate can be made by mixing solutions of calcium chloride and sodium carbonate. The white precipitate of calcium carbonate is then filtered, washed and dried.

Model answer (4 marks)

The actual yield is less than the theoretical yield because the theoretical yield is the maximum mass of product that can be obtained if the reaction goes to completion and all product is recovered.

Two reasons why the yield is lower are:
1. Some of the precipitate is lost during transfer – for example, it may remain stuck to the filter paper or on the glassware.
2. The reaction may be incomplete or a side reaction may occur, so not all of the reactants are converted to calcium carbonate.

Examiner tips

  • Use the word ‘because’ to link cause and effect; include the definition of theoretical yield. Mention both loss during filtration and incomplete reaction/side reaction as separate points. Keep each point concise and use the exact terminology (e.g., ‘precipitate’, ‘filter paper’).

Common mistakes

  • Writing that the yield is lower because the reaction is ‘slow’ rather than incomplete or due to loss. Forgetting to mention the theoretical yield definition. Using vague terms like ‘some product is lost’ without specifying filtration or side reactions.

Mark scheme (4 marks)

  1. The actual yield is less than the theoretical yield because the theoretical yield is the maximum possible mass of product that can be obtained from a reaction (and in practice this is rarely achieved).
  2. Some product is lost during transfer, e.g. some precipitate remains stuck to the filter paper or glassware.
  3. The reaction may be incomplete, meaning not all reactants have reacted to form product.
  4. A side reaction may occur, producing a by-product instead of the desired calcium carbonate, reducing the amount of useful product formed.

Key terms in this question

actual yield · theoretical yield

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