Showing that air contains about one-fifth oxygen

4 marks · Normal

Copper can be used to determine the percentage of oxygen in air.

(a)

Describe the main steps in the experiment using heated copper to find the percentage of oxygen in air.

[2 marks]

(b)

Explain why excess copper is needed in this experiment.

[1 mark]

(c)

Suggest one safety precaution for this practical.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) Air is passed repeatedly over heated copper until there is no further decrease in gas volume; the final volume is measured after cooling. · must show Pass air over heated copper, Measure initial and final volume, Allow apparatus to cool before recording final volume

(b) To ensure all the oxygen reacts and is removed from the air. · must show Excess required so all oxygen is reacted/removed

(c) Wear eye protection or handle hot apparatus carefully. · must show Any valid safety such as eye protection, careful heating

Common mistakes: Not mentioning cooling before taking final volume; Not describing passing air repeatedly over copper

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · Describes passing air over heated copper multiple times / both syringes used in sequence.

M2 (a) · 1 mark · States volumes measured before and after, and/or apparatus is cooled before reading final volume.

B1 (b) · 1 mark · Explains excess copper: ensures all oxygen reacts/is removed from air.

B2 (c) · 1 mark · Gives relevant safety precaution (e.g. eye protection, heat carefully, do not push plungers out).

Worked solution

a) Air is passed repeatedly over heated copper in a silica tube by pushing syringes in and out. The volume of air is measured at the start and at the end, after the apparatus is cooled to room temperature.

b) Excess copper is used so all the oxygen in the air reacts. If not, not all oxygen would be removed and the result would be too low.

c) Wear eye protection during heating and take care when handling hot equipment.