# A student burns magnesium in excess oxygen. The reaction produces magnesium oxide. The student notices that the mass of the product is greater than the mass of magnesium used at the start. Explain why the mass of magnesium oxide formed is greater than the mass of magnesium used, and explain what is meant by the term 'limiting reactant' in this reaction.

> Eduqas A-Level Chemistry — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks

> When magnesium burns in excess oxygen, the balanced equation for the reaction is: 2Mg + O₂ → 2MgO

## Mark scheme (5 marks)

1. No atoms are lost or made during a chemical reaction (conservation of mass)
2. The magnesium atoms combine with oxygen atoms from the air/oxygen gas
3. The mass of magnesium oxide equals the mass of magnesium plus the mass of oxygen that reacted
4. The limiting reactant is the reactant that is completely used up
5. In this reaction, magnesium is the limiting reactant because it limits the amount of magnesium oxide (product) formed / oxygen is in excess

## Key terms

- [limiting reactant](https://www.gradenine.co.uk/glossary/limiting-reactant)
- [excess](https://www.gradenine.co.uk/glossary/excess)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-burns-magnesium-in-excess-bc291430) · Published by Druglandscape Ltd.