# Calcium reacts with oxygen to form calcium oxide. A student claims that the mass of calcium oxide produced must equal the total mass of calcium and oxygen that reacted. Explain why the student is correct, and describe what an empirical formula tells a chemist about a compound.

> Eduqas A-Level Chemistry — 3.1 Introduction to organic chemistry · Explain · 5 marks

## Mark scheme (5 marks)

1. No atoms are lost or made during a chemical reaction
2. Therefore the mass of the products equals the mass of the reactants (law of conservation of mass)
3. An empirical formula gives the simplest whole number ratio of atoms of each element in a compound
4. The empirical formula of calcium oxide is CaO, showing a 1:1 ratio of calcium to oxygen atoms
5. The empirical formula may differ from the molecular formula, which shows the actual number of atoms present in a molecule

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-reacts-with-oxygen-to-form-c39c7613) · Published by Druglandscape Ltd.