A student reacts magnesium with hydrochloric acid to produce magnesium chloride. The theoretical yield of magnesium chloride is 19.0 g. The actual yield obtained is 13.3 g. Explain why the actual yield is less than the theoretical yield, and describe two steps the student could take during the experiment to improve the percentage yield.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Percentage yield is calculated as: (actual yield ÷ theoretical yield) × 100. The student obtains a percentage yield of approximately 70%.
Model answer (4 marks)
The actual yield is less than the theoretical yield because the reaction may be incomplete – not all of the magnesium reacts with the acid to form magnesium chloride.
The product can also be lost during the experiment, for example when transferring the solution between containers, during filtration or when evaporating the solvent.
To improve the yield the student could:
1. Rinse all glassware with distilled water (or use a wash bottle) to recover any magnesium chloride that remains on the walls of the containers.
2. Ensure the reaction goes to completion, for example by using an excess of hydrochloric acid or by allowing the reaction to run for a sufficient time before isolating the product.
The product can also be lost during the experiment, for example when transferring the solution between containers, during filtration or when evaporating the solvent.
To improve the yield the student could:
1. Rinse all glassware with distilled water (or use a wash bottle) to recover any magnesium chloride that remains on the walls of the containers.
2. Ensure the reaction goes to completion, for example by using an excess of hydrochloric acid or by allowing the reaction to run for a sufficient time before isolating the product.
Examiner tips
- Use the exact phrase "incomplete reaction" and "product loss" – these are the terms the mark scheme looks for. Show the two specific steps clearly and link each to the loss mechanism. Keep the answer concise – 4 short sentences is enough for full marks.
Common mistakes
- Writing that the yield is less because of a mistake in weighing the product – this is not what the mark scheme rewards. Failing to mention that the reaction may be incomplete or that product can be lost during transfer. Giving a step that does not actually reduce product loss, e.g. "cool the solution" – this is irrelevant.
Mark scheme (4 marks)
- The actual yield is less than the theoretical yield because the reaction may be incomplete / not all the reactants are converted to products
- Product can be lost during transfer between containers / during filtration / evaporation steps
- One valid improvement: use a wash bottle / rinse all equipment with distilled water to recover any product left on the sides of the container
- Second valid improvement: ensure the reaction is allowed to go to completion before collecting the product / use excess of one reactant to ensure the other is fully used up
Key terms in this question
actual yield · theoretical yield · percentage yield
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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