# A chemist is investigating a sample of a compound to determine its empirical formula. The compound contains only carbon and hydrogen. The chemist finds that 3.6 g of the compound contains 3.0 g of carbon and 0.6 g of hydrogen. Explain how the empirical formula of this compound is determined from this data, and state what the empirical formula tells us about the compound.

> OCR A-Level Chemistry B: Salters (H433) — 2.2 Amount of substance · Explain · 5 marks

> A compound containing only carbon and hydrogen has a mass of 3.6 g. Of this, 3.0 g is carbon and 0.6 g is hydrogen. The relative atomic mass of carbon is 12 and the relative atomic mass of hydrogen is 1.

## Mark scheme (5 marks)

1. Divide the mass of each element by its relative atomic mass to find the number of moles (ratio) of each element
2. Carbon gives 3.0 ÷ 12 = 0.25 (moles) and hydrogen gives 0.6 ÷ 1 = 0.6 (moles)
3. Divide both values by the smallest value (0.25) to obtain the simplest whole number ratio
4. Correct empirical formula stated as CH2 (ratio C:H = 1:2.4, multiply by 5 gives C5H12, but simplest whole number ratio from 1:2.4 → multiply by 5 gives C5:H12, so empirical formula is C5H12) — accept CH2 if ratio correctly derived as 1:2.4 rounded/multiplied to smallest integers giving C5H12
5. The empirical formula shows 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

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