A student dissolves iron sulfide in excess dilute sulfuric acid inside a sealed container placed on a balance. The reading on the balance does not change during the reaction. Explain why the mass stays the same, and explain what would happen to the balance reading if the container were left open during the reaction.

AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · 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 reaction is:
FeS(s)+2H⁺(aq)→Fe²⁺(aq)+H₂S(g)

1. In a sealed container all reactants and products remain inside, so no atoms leave the system.
2. By the law of conservation of mass the total mass of FeS, H₂S and the acid before the reaction equals the total mass of Fe²⁺, H₂S and the acid after the reaction.
3. Therefore the balance reading does not change.
4. If the container were open, the H₂S gas would escape into the atmosphere.
5. The escaped gas is no longer on the balance, so the measured mass would decrease.

Examiner tips

  • Use the law of conservation of mass to justify the unchanged reading; mention the sealed system. Then state that opening the container lets H₂S escape and the balance falls. Keep the answer concise and use the exact reaction.
  • Show the reaction equation to demonstrate the gas product.
  • Explain that the gas leaving the system is the reason for the mass loss.

Common mistakes

  • Failing to recognise that H₂S is a gas and would escape if the container is open.
  • Confusing the sealed system with an open one and not linking the escape of gas to a decrease in mass.

Mark scheme (4 marks)

  1. No atoms are lost or made during a chemical reaction (law of conservation of mass)
  2. So the total mass of products equals the total mass of reactants
  3. In an open container a gas (hydrogen sulfide) would escape / be released into the atmosphere
  4. The balance reading would decrease because the mass of the escaped gas is not measured

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