# 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

> 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.

## 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

- [rate of reaction](https://www.gradenine.co.uk/glossary/rate-of-reaction)
- [loss in mass](https://www.gradenine.co.uk/glossary/loss-in-mass)
- [cotton wool plug](https://www.gradenine.co.uk/glossary/cotton-wool-plug)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain and Evaluate" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-rate-536ec12d) · Published by Druglandscape Ltd.