# A student analyses a compound containing only iron and sulfur. The student finds that 2.24 g of iron combines with 1.92 g of sulfur to form the compound. Use the relative atomic masses Fe = 56, S = 32 to determine the empirical formula of the compound. Show your reasoning at each step.

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

> Iron sulfide compounds are used in the manufacture of certain pigments. Determining the empirical formula of such compounds is an important analytical task in the chemical industry.

## Mark scheme (5 marks)

1. Calculates the number of moles of iron: 2.24 ÷ 56 = 0.04 mol
2. Calculates the number of moles of sulfur: 1.92 ÷ 32 = 0.06 mol
3. Divides both values by the smallest molar amount (0.04) to find the ratio
4. Recognises that a ratio of 1 : 1.5 must be multiplied by 2 to give whole numbers, giving Fe : S = 2 : 3
5. States the correct empirical formula: Fe₂S₃

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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-analyses-a-compound-containing-cf994a25) · Published by Druglandscape Ltd.