# Magnesium carbonate decomposes on heating according to the equation: MgCO₃ → MgO + CO₂. The relative formula mass of MgCO₃ is 84 and the relative formula mass of MgO is 40. A student heats a sample of magnesium carbonate and notices that the solid left in the crucible has a lower mass than the original sample. Explain why the mass decreases and use the relative formula masses to describe the relationship between the mass of magnesium carbonate that reacts and the mass of magnesium oxide produced.

> AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks

## Mark scheme (4 marks)

1. Carbon dioxide (CO₂) is produced as a gas and escapes / is released into the atmosphere
2. The law of conservation of mass states that no atoms are lost or made, so the total mass of products equals the mass of reactants
3. 84 g of magnesium carbonate produces 40 g of magnesium oxide (ratio of Mr values used correctly)
4. The remaining 44 g (of mass) is accounted for by the carbon dioxide released / mass of CO₂ = 84 − 40 = 44

## Key terms

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

## Related

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