The combustion of elements in oxygen
2 marks · EasyExplain why it is important to leave some phosphorus unreacted at the end of the experiment when finding the percentage of oxygen in air using phosphorus.
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So that all the oxygen reacts and ensures an accurate measurement—if all the phosphorus is used up, some oxygen may remain, giving a less accurate (lower) result.
Must show: Explains excess phosphorus ensures all oxygen reacts, Mentions accuracy or correct percentage result
Common mistakes: states it prevents phosphorus wastage, not accuracy; misses that oxygen may remain
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Marking points
M1 · 1 mark · Explains unreacted phosphorus ensures all oxygen has reacted
M1 · 1 mark · Links to accuracy/correct measurement of oxygen percentage
Worked solution
If all the phosphorus is used up, there could still be some oxygen remaining, so the measurement of oxygen would be too low. Leaving excess phosphorus ensures all the oxygen reacts, making the result accurate.