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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The law of conservation of mass states that no atoms are lost or made during a chemical reaction. In the combustion of a hydrocarbon, the mass of the products equals the mass of the reactants. All the carbon atoms from the hydrocarbon end up in the carbon dioxide produced, and all the hydrogen atoms from the hydrocarbon end up in the water produced. 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).
Examiner tips
- Use the exact phrase ‘law of conservation of mass’
- Show the chain of reasoning: atoms → products → mass equality
- Mention that all C atoms go to CO₂ and all H atoms to H₂O
- Keep the answer concise and to the point
Common mistakes
- Confusing conservation of mass with conservation of energy
- Omitting the statement that all C atoms end up in CO₂ and all H atoms in H₂O
- Using vague wording such as ‘atoms are conserved’ without linking to mass equality
Mark scheme (5 marks)
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction
- The mass of the products equals the mass of the reactants
- All the carbon atoms from the hydrocarbon end up in the carbon dioxide produced
- All the hydrogen atoms from the hydrocarbon end up in the water produced
- 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 in this question
conservation of mass · products · hydrocarbon
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