A student prepares magnesium oxide by burning magnesium in air. The theoretical yield of magnesium oxide is 25 g. The student collects only 18 g of magnesium oxide. Explain why the actual yield is lower than the theoretical yield, and describe what is meant by percentage yield.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium burns in air to produce magnesium oxide. The theoretical yield of magnesium oxide from a given mass of magnesium is 25 g. The student collects 18 g of magnesium oxide.
Model answer (4 marks)
The theoretical yield is the maximum amount of product that could be produced if the reaction went to completion and no product was lost. The actual yield is lower because the reaction may not go to completion, some magnesium oxide may be lost during collection, or side reactions may consume part of the reactant. Percentage yield is the ratio of the actual yield to the theoretical yield, expressed as a percentage. A 100 % yield would mean all of the theoretical product was obtained; a lower percentage indicates that some product was not recovered.
Examiner tips
- Define theoretical and actual yield first; explain loss mechanisms; state the formula for % yield; give an example of 100 % vs <100 %
Common mistakes
- Confusing theoretical yield with actual yield; not explaining why the yield is lower; writing % yield as a fraction instead of a percentage
Mark scheme (4 marks)
- The theoretical yield is the maximum amount of product that could be produced, assuming a complete reaction takes place
- The actual yield is lower than the theoretical yield because the reaction may not go to completion / some product may be lost during collection / side reactions may occur
- Percentage yield is the ratio of the actual yield compared to the theoretical yield (expressed as a percentage)
- A percentage yield of 100% would mean all the theoretical yield was obtained / a percentage yield less than 100% means some product was not collected
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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