# A student wants to find the empirical formula of a compound containing only carbon and hydrogen. The compound is found to contain 75% carbon by mass. The rest of the mass is hydrogen. Use this information to determine the empirical formula of the compound. Show your reasoning clearly, stating the steps you would take.

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

> A compound contains only carbon and hydrogen. Analysis shows that 75% of its mass is carbon and the remaining 25% is hydrogen. The relative atomic mass of carbon is 12 and the relative atomic mass of hydrogen is 1.

## Mark scheme (5 marks)

1. Assume a 100 g sample so that the percentages can be treated as masses in grams: 75 g of carbon and 25 g of hydrogen
2. Divide the mass of carbon by its relative atomic mass: 75 ÷ 12 = 6.25 moles of carbon
3. Divide the mass of hydrogen by its relative atomic mass: 25 ÷ 1 = 25 moles of hydrogen
4. Divide both molar amounts by the smallest value (6.25) to find the simplest whole number ratio: carbon = 1, hydrogen = 4
5. States the empirical formula as CH₄

## 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 "Describe" 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-wants-to-find-the-2ae02865) · Published by Druglandscape Ltd.