# A student reacts 24 g of magnesium with excess hydrochloric acid. The equation for the reaction is: Mg + 2HCl → MgCl₂ + H₂. Explain, using the concept of moles, why the amount of hydrogen gas produced is the same number of moles as the magnesium used. Then state what mass of hydrogen gas would be produced. (Relative atomic masses: Mg = 24, H = 1)

> AQA A-Level Chemistry (7405) — 3.1.2 Amount of substance · Explain · 5 marks

## Mark scheme (5 marks)

1. Moles of Mg = mass ÷ relative atomic mass = 24 ÷ 24 = 1 mol
2. The mole ratio of Mg to H₂ in the balanced equation is 1:1
3. Therefore moles of H₂ produced = 1 mol
4. Relative molecular mass of H₂ = 2
5. Mass of H₂ = 1 × 2 = 2 g

## Key terms

- [moles](https://www.gradenine.co.uk/glossary/moles)
- [relative atomic mass](https://www.gradenine.co.uk/glossary/relative-atomic-mass)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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-reacts-24-g-of-50e6cd94) · Published by Druglandscape Ltd.