# A chemist is investigating an unknown ionic compound. The compound contains only magnesium and oxygen. The chemist determines that the compound contains 60.3% magnesium and 39.7% oxygen by mass. Use this information to determine the empirical formula of the compound, showing your reasoning clearly.

> OCR A-Level Chemistry A (H432) — 2.2 Amount of substance · Justify · 5 marks

> Magnesium has a relative atomic mass of 24.3. Oxygen has a relative atomic mass of 16.0.

## Mark scheme (5 marks)

1. Divides percentage of magnesium by its relative atomic mass: 60.3 ÷ 24.3
2. Divides percentage of oxygen by its relative atomic mass: 39.7 ÷ 16.0
3. Divides both values by the smallest to find the simplest ratio
4. States the correct empirical formula as MgO
5. Correctly defines empirical formula as the simplest whole number ratio of atoms of each element in the compound

## Key terms

- [empirical formula](https://www.gradenine.co.uk/glossary/empirical-formula)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Justify" 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-chemist-is-investigating-an-unknown-8d5ddf0c) · Published by Druglandscape Ltd.