A student carries out a reaction to produce silver chloride as a precipitate. The theoretical yield of silver chloride is 40.0 g, but the student only collects 28.0 g. Explain why the actual yield is less than the theoretical yield, and describe two practical 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).

Silver chloride is an insoluble white solid that can be produced by mixing solutions of silver nitrate and sodium chloride. It is used in photography and specialist glass manufacture.

Model answer (4 marks)

The actual yield is less than the theoretical yield because the theoretical yield is the maximum possible mass of product that can be formed if all of the limiting reactant is completely converted into product. In practice, not all of the silver nitrate and sodium chloride are converted into silver chloride.

Two practical reasons for the loss are:
1. Some of the precipitate is lost during transfer, filtering or washing of the solid.
2. Side reactions or competing reactions consume some of the reactants, forming unwanted by‑products and reducing the amount of silver chloride produced.

The percentage yield is therefore less than 100 % (actual ÷ theoretical × 100 = 70 %).

Examiner tips

  • Use the word ‘theoretical yield’ to explain the maximum possible mass; mention incomplete conversion. Show the two practical reasons clearly, using terms like ‘lost during filtering’ and ‘side reactions’. Include the calculation of the percentage yield to demonstrate understanding of the concept.

Common mistakes

  • Confusing theoretical yield with the amount of product actually obtained. Forgetting to mention that the yield is less than 100 %. Using vague phrases such as ‘something went wrong’ instead of specific practical reasons.

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 from a reaction / not all reactants are converted to product
  2. First practical reason: some precipitate is lost during transfer / filtering / washing of the solid
  3. Second practical reason: side reactions / competing reactions produce unwanted by-products, reducing the amount of desired product formed
  4. Percentage yield is less than 100% — calculated as (actual yield ÷ theoretical yield) × 100 / the student's percentage yield is 70%

Key terms in this question

actual yield · theoretical yield · precipitate

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