Showing that air contains about one-fifth oxygen
4 marks · NormalThe table shows results from an experiment to determine the composition of air.
| Gas | Estimated percentage (%) |
|---|---|
| Oxygen | 20 |
| Nitrogen | 75 |
| Argon | 1 |
| Carbon dioxide | 0.04 |
Compare the estimated percentage of nitrogen and oxygen to their approximate true values in dry air.
Give two possible reasons why the estimated results might differ from the true values.
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(a) Nitrogen is estimated lower than true value (75% vs 78%); oxygen is estimated lower than true value (20% vs 21%).
(b) Experimental error (such as incomplete reaction), or inaccurate volume readings; or not enough reagent / experiment not left long enough.
Common mistakes: Incomplete comparison; Only giving one reason
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Marking points
A1 (a) · 1 mark · Identifies nitrogen is estimated lower than true value (75% vs 78%).
A2 (a) · 1 mark · Identifies oxygen is estimated lower than true value (20% vs 21%).
B1 (b) · 1 mark · States one valid reason for difference (e.g., experiment not complete, reagent not in excess, gas syringe error).
B2 (b) · 1 mark · States a second valid reason (accept experimental limitations, leaks, incorrect readings).
Worked solution
a) Nitrogen and oxygen estimated are both a little lower than the approximate true values (nitrogen: 75% vs 78%; oxygen: 20% vs 21%).
b) Possible reasons: Not all oxygen reacted because the experiment was not left long enough; volume readings might have been inaccurate; reagents (copper or iron) might not have been in excess; or some gas may have leaked during the experiment.