A student pushes 120 cm³ of air backwards and forwards over heated copper turnings using two gas syringes. The copper reacts with oxygen. When the apparatus has cooled, the volume of gas remaining is 95 cm³.
(a) Calculate the percentage of oxygen in this sample of air. Give your answer to 3 significant figures. [2]
(b) State the colour change of the copper. [1]
(c) Explain why the gas must be allowed to cool before the final volume is measured. [1]
(d) Name the main gas left in the syringe. [1]
- (a) 120 − 95 = 25 (cm³) of oxygen used(1)
- (a) 25 ÷ 120 × 100 = 20.8 (%) (correct answer with no working scores 2 marks; 20.83 or 21 without 20.8 scores M1 only, as 3 s.f. is asked for)(1)
- (b) (pink / orange-brown / red-brown / brown) to black(1)
- (c) gas expands when hot / hot gas has a larger volume (so the reading would be too high) (allow so the final volume is measured at the same temperature as the starting volume / room temperature)(1)
- (d) nitrogen (allow N₂; ignore argon / noble gases given in addition; reject nitrogen oxides)(1)
- Answer: (a) 20.8% (b) pink-brown to black (c) gas contracts on cooling (d) nitrogen
- Examiner tip: Divide by the STARTING volume (120 cm³), not the final volume.