Explain how the law of conservation of mass applies to a chemical reaction carried out in a closed container, and explain why the measured mass appears to decrease when a reaction produces a gas in an open container.

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

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

Model answer (5 marks)

In a closed container the law of conservation of mass is obeyed: the total mass of the reactants equals the total mass of the products because atoms are merely rearranged and none are created or destroyed.

In an open container a gas may be produced. The gas leaves the reaction vessel and is released into the surroundings. Consequently the mass that has escaped is no longer on the balance and the measured mass appears to decrease. Mass is still conserved overall – if the gas were collected the total mass would remain unchanged.

Examiner tips

  • Use the phrase "law of conservation of mass" and state "total mass of reactants equals total mass of products" for the closed system.
  • Explain that atoms are rearranged and none are created or destroyed.
  • Mention that in an open system the gas escapes, so the mass is not measured.
  • Show that overall mass is still conserved if the gas were collected.

Common mistakes

  • Confusing the closed and open container situations, or saying mass increases in the closed system.
  • Failing to mention that atoms are merely rearranged, or that the gas simply escapes the vessel.

Mark scheme (5 marks)

  1. In a closed container, mass is conserved / total mass of reactants equals total mass of products
  2. Atoms are rearranged / no atoms are created or destroyed during a chemical reaction
  3. In an open container, when a gas is produced it escapes / is released into the surroundings
  4. The mass of the gas that escapes is no longer on the balance / is not measured
  5. Mass is still conserved overall / if the gas were collected the total mass would be unchanged

Key terms in this question

law of conservation of mass · closed container · open container · gas

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