Showing that air contains about one-fifth oxygen
2 marks · EasyExplain why it is important to use excess copper in the experiment to determine the percentage of oxygen in air using copper.
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To ensure all the oxygen reacts with the copper; if not enough copper is used, not all the oxygen will be removed, and the calculated percentage of oxygen will be too low.
Also accept: So all oxygen is removed; otherwise the result would underestimate the oxygen percentage
Common mistakes: Saying excess copper increases the oxygen amount; Excess copper speeds up reaction
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Marking points
M1 · 1 mark · States that excess copper ensures all the oxygen reacts/is removed
E1 · 1 mark · Mentions that not using enough copper leads to a lower (too low) value for oxygen percentage
Worked solution
Excess copper ensures that all the oxygen in the air reacts. If not enough copper is used, not all the oxygen will be removed, so the calculated percentage of oxygen will be too low.