A student is investigating the rate of reaction between zinc powder and sulfuric acid by measuring the loss in mass of the reaction mixture over time. The student places a conical flask containing sulfuric acid on a balance and records the starting mass. They add zinc powder, loosely place a cotton wool plug in the neck of the flask, and record the mass every 30 seconds for six minutes. Explain how the student can use this method to determine the rate of reaction, and evaluate one advantage and one disadvantage of using a cotton wool plug in this investigation.

OCR A-Level Chemistry A (H432) — 1.1 Practical skills assessed · Explain and Evaluate · 5 marks · View as Markdown

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

Zinc powder reacts with sulfuric acid to produce zinc sulfate and hydrogen gas. The reaction can be followed by monitoring the loss in mass of the reaction mixture as gas escapes.

Model answer (5 marks)

The loss in mass of the conical flask is due to hydrogen gas leaving the system. By recording the mass every 30 s, the student can plot mass (g) against time (s). The slope of the initial part of the curve (Δmass/Δtime) gives the rate of mass loss, which is proportional to the rate of reaction. When the mass stops changing, the reaction is complete.

Advantage of the cotton‑wool plug: it permits hydrogen to escape while preventing acid spray, so the mass change can be measured accurately and the experiment is safer.

Disadvantage of the cotton‑wool plug: the cotton absorbs some hydrogen and acid mist, so part of the gas is trapped. This reduces the measured mass loss and makes the rate appear lower than the true value.

Examiner tips

  • Show the mass–time graph and state that the slope gives the rate; mention that a constant mass indicates completion.
  • Explain the plug’s dual role of allowing gas out and stopping spray; note the absorption error as a drawback.

Common mistakes

  • Confusing the loss in mass with the amount of acid used;
  • Failing to recognise that the plug can absorb gas and thus underestimate the rate;
  • Not linking the slope of the mass–time graph to the rate of reaction.

Mark scheme (5 marks)

  1. The loss in mass over time is due to hydrogen gas escaping from the flask.
  2. The rate of reaction can be determined by calculating how quickly the mass decreases (e.g. the greater the loss in mass per unit time, the faster the rate).
  3. The reaction is complete / has finished when the mass stops decreasing (the reading becomes constant).
  4. Advantage of cotton wool plug: it allows hydrogen gas to escape (so the mass change can be recorded) while preventing acid spray / droplets from leaving the flask, making the results more accurate / preventing a hazard.
  5. Disadvantage of cotton wool plug: the cotton wool itself absorbs some hydrogen gas / acid mist, meaning some gas is trapped and the recorded mass loss is smaller than the actual amount of gas produced, reducing accuracy.

Key terms in this question

rate of reaction · loss in mass · cotton wool plug

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