# A geologist discovers a mineral sample and wants to determine the empirical formula of the iron oxide it contains. The mineral is found to contain only iron and oxygen. Explain how the geologist could use the masses of iron and oxygen in the compound to determine its empirical formula.

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

> When a sample of the iron oxide is analysed, it is found to contain 2.24 g of iron and 0.96 g of oxygen. The relative atomic masses are: Fe = 56, O = 16.

## Mark scheme (5 marks)

1. Divide the mass of each element by its relative atomic mass to find the number of moles of each element
2. Correctly calculates moles of iron as 0.04 mol (2.24 ÷ 56)
3. Correctly calculates moles of oxygen as 0.06 mol (0.96 ÷ 16)
4. Divide both values by the smallest number of moles to find the simplest whole number ratio
5. States the empirical formula is Fe₂O₃, giving the ratio Fe : O = 1 : 1.5 → 2 : 3

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-geologist-discovers-a-mineral-sample-b5ea9acb) · Published by Druglandscape Ltd.