A student carries out a reaction to produce copper sulfate. The theoretical yield of copper sulfate is 20 g, but the student only collects 15 g. Explain why the actual yield is less than the theoretical yield, giving two reasons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In industry and in the laboratory, chemical reactions rarely produce the maximum possible amount of product.
Model answer (4 marks)
The actual yield is less than the theoretical yield because:
1. Some product is lost during the practical work – for example, during filtering, transferring or evaporating the copper sulfate.
2. The reaction is reversible, so it does not go to completion and not all of the reactants are converted into product.
The percentage yield is calculated as (actual yield ÷ theoretical yield) × 100 = (15 g ÷ 20 g) × 100 = 75 %.
1. Some product is lost during the practical work – for example, during filtering, transferring or evaporating the copper sulfate.
2. The reaction is reversible, so it does not go to completion and not all of the reactants are converted into product.
The percentage yield is calculated as (actual yield ÷ theoretical yield) × 100 = (15 g ÷ 20 g) × 100 = 75 %.
Examiner tips
- Use the exact wording from the mark scheme – mention loss during practical work and reversibility. Show the percentage‑yield calculation with the correct formula and units. Keep the answer concise and to the point.
- common_mistakes
- :
- Writing reasons that are too vague (e.g. ‘inefficiency’) instead of the specific practical loss and reversibility. Forgetting to calculate the percentage yield or using the wrong formula.
Mark scheme (4 marks)
- The actual yield is less than the theoretical yield because some product may be lost during the practical process (e.g. during filtering, transferring or evaporating)
- The reaction may be reversible, so it does not go to completion and not all reactants are converted to products
- Percentage yield is calculated by dividing the actual yield by the theoretical yield and multiplying by 100
- The percentage yield in this case is 75%
Key terms in this question
actual yield · theoretical yield
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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