# A student wants to find the empirical formula of a compound containing only carbon, hydrogen, and oxygen. She burns a sample of the compound completely in excess oxygen. The products are carbon dioxide and water only. Explain how the student could use the masses of carbon dioxide and water produced to determine the empirical formula of the compound.

> AQA A-Level Chemistry (7405) — 3.1.2 Amount of substance · Explain · 5 marks

> The empirical formula of a compound is the simplest whole-number ratio of atoms of each element in the compound.

## Mark scheme (5 marks)

1. Calculate the number of moles of carbon dioxide produced (moles = mass ÷ relative formula mass / moles = mass ÷ 44)
2. Each mole of carbon dioxide contains one mole of carbon atoms, so moles of carbon = moles of CO₂
3. Calculate the number of moles of water produced (moles = mass ÷ 18), and moles of hydrogen atoms = 2 × moles of water
4. Find moles of oxygen by subtracting (moles of C × 16) and (moles of H × 1) from the original mass of the compound, then divide by 16
5. Divide all three values of moles by the smallest value to get the simplest whole-number ratio, giving the empirical formula

## Key terms

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

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-wants-to-find-the-7ec0ecd6) · Published by Druglandscape Ltd.