# A student burns a hydrocarbon fuel completely in excess oxygen. The only products formed are carbon dioxide and water. Explain what the law of conservation of mass means and use it to explain why the total mass of carbon dioxide and water produced must equal the mass of the hydrocarbon and oxygen that reacted.

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

> A hydrocarbon is a compound that contains carbon and hydrogen only. When a hydrocarbon burns completely in excess oxygen, carbon dioxide and water are the only products formed.

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. The mass of the products equals the mass of the reactants
3. All the carbon atoms from the hydrocarbon end up in the carbon dioxide produced
4. All the hydrogen atoms from the hydrocarbon end up in the water produced
5. Because no atoms are gained or lost, the total mass of carbon dioxide and water (products) must equal the total mass of hydrocarbon and oxygen (reactants)

## Key terms

- [conservation of mass](https://www.gradenine.co.uk/glossary/conservation-of-mass)
- [products](https://www.gradenine.co.uk/glossary/products)
- [hydrocarbon](https://www.gradenine.co.uk/glossary/hydrocarbon)

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" 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-burns-a-hydrocarbon-fuel-4f84981e) · Published by Druglandscape Ltd.