# 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.

> AQA GCSE Chemistry (8462) — 4.3.3 Yield and atom economy of chemical reactions · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. The theoretical yield is the maximum amount of product that could be produced, assuming a complete reaction takes place
2. 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
3. Percentage yield is the ratio of the actual yield compared to the theoretical yield (expressed as a percentage)
4. 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

- [actual yield](https://www.gradenine.co.uk/glossary/actual-yield)
- [theoretical yield](https://www.gradenine.co.uk/glossary/theoretical-yield)
- [percentage yield](https://www.gradenine.co.uk/glossary/percentage-yield)

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-prepares-magnesium-oxide-by-533c9c06) · Published by Druglandscape Ltd.