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Gas collection volume discrepancy
A student investigated the rate of the reaction between zinc and sulfuric acid. The equation for the reaction is:
Figure 10 shows the apparatus.
This is the method used.
- Pour 50 cm³ of sulfuric acid into the conical flask.
- Add excess zinc to the conical flask.
- Insert the stopper and start a timer.
- Measure the volume of hydrogen collected in the 50 cm³ measuring cylinder every 20 seconds for 180 seconds.
Explain why the volume of hydrogen collected in the 50 cm³ measuring cylinder is less than the volume of hydrogen produced.
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Marking points
- 1 States that some hydrogen escapes from the flask before collection.
- 2 Explains that the reaction begins before the stopper can be inserted.
Full-mark answer
The reaction starts as soon as zinc is added, before the stopper can be inserted. Some hydrogen therefore escapes from the flask instead of entering the measuring cylinder.
Why this answer loses marks
The cylinder collects less gas because it is too small.
The loss happens before collection: the reaction starts before the stopper is inserted, allowing hydrogen to escape.