A student carries out a reaction to produce copper sulfate crystals. The theoretical yield of copper sulfate is 25.0 g, but the student only collects 18.5 g of crystals. 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 practical.

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).

Percentage yield is used to compare the actual yield of a product with the theoretical yield. The actual yield is almost always less than the theoretical yield.

Model answer (4 marks)

The actual yield is less than the theoretical yield because the reaction is not 100 % efficient – not all of the reactant is converted into product, so the amount of copper sulfate formed is lower than the maximum possible.

Two reasons for this loss are:
1. Some of the product is lost during transfer, e.g. when the crystals are filtered or transferred between beakers.
2. Side reactions can occur, consuming reactant and forming by‑products instead of copper sulfate.

The percentage yield is calculated as (18.5 g ÷ 25.0 g) × 100 = 74 % – less than 100 % because the actual yield is lower than the theoretical yield.

Examiner tips

  • Explain the general reason first, then give two specific practical causes; use the word ‘theoretical’ and ‘actual’ correctly.
  • Show the percentage yield calculation if asked, but here focus on the explanation and reasons.

Common mistakes

  • Using vague terms like ‘inefficient’ without linking to incomplete conversion; Failing to mention both transfer loss and side reactions; Not recognising that the percentage yield is a consequence of the lower actual yield.

Mark scheme (4 marks)

  1. The actual yield is less than the theoretical yield because not all of the reactant is converted into product / the reaction may be incomplete
  2. Some product may be lost during transfer between containers / filtering / evaporating
  3. Side reactions may occur, producing by-products instead of the desired product
  4. The percentage yield is less than 100% because the actual yield (18.5 g) is less than the theoretical yield (25.0 g), showing that not all possible product was obtained

Key terms in this question

actual yield · theoretical yield

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