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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The mass decreases because carbon dioxide is released as a gas and leaves the crucible.

The law of conservation of mass means the total mass of products equals the mass of reactants.

From the relative formula masses, 84 g MgCO₃ gives 40 g MgO.

The remaining 44 g (84−40) is the CO₂ that escapes, so the mass loss equals the mass of CO₂ released.

Examiner tips

  • Show the gas escape first to get the 1st mark. Use the law of conservation of mass to justify the relationship. Calculate 84−40=44 to link masses. Mention the 44 g as CO₂ to get the final mark.

Common mistakes

  • Forgetting that CO₂ is a gas and leaves the crucible. Using the wrong relative masses (e.g. 40 g MgCO₃ to 84 g MgO). Not showing the calculation 84−40=44.

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 in this question

relative formula mass

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