A student reacts zinc with excess hydrochloric acid to produce zinc chloride. The theoretical yield of zinc chloride is 40 g. The student collects 30 g of zinc chloride. Explain why the actual yield is less than the theoretical yield, and calculate the percentage yield.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc reacts with hydrochloric acid according to the following equation: Zn + 2HCl → ZnCl₂ + H₂. The theoretical yield of zinc chloride is 40 g and the student collects an actual yield of 30 g.
Model answer (4 marks)
The actual yield is lower than the theoretical yield because:
1. Some zinc chloride may have been lost during filtration or transfer, for example by sticking to the filter paper or evaporating.
2. The reaction may not have gone to completion, leaving unreacted zinc or hydrogen gas, so less product is formed.
Percentage yield = (30 g ÷ 40 g) × 100 = 0.75 × 100 = 75 %.
1. Some zinc chloride may have been lost during filtration or transfer, for example by sticking to the filter paper or evaporating.
2. The reaction may not have gone to completion, leaving unreacted zinc or hydrogen gas, so less product is formed.
Percentage yield = (30 g ÷ 40 g) × 100 = 0.75 × 100 = 75 %.
Examiner tips
- Use the word ‘because’ to link each reason to the lower yield; give two distinct, realistic reasons. Show the percentage‑yield formula and insert the numbers before calculating. State the final percentage clearly, e.g. ‘75 %’.
- common_mistakes
- :
- Writing a single reason instead of two. Using the wrong formula, e.g. (theoretical ÷ actual) × 100. Failing to include the percent sign or giving a decimal instead of a percentage.
Mark scheme (4 marks)
- Identifies a valid reason why actual yield is less than theoretical yield
- Identifies a second valid reason why actual yield is less than theoretical yield
- Correct method: percentage yield = (actual yield ÷ theoretical yield) × 100
- Correct final answer: 75%
Key terms in this question
actual yield · theoretical yield · percentage yield
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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