the question

3. Peter performed an experiment to determine the percentage of oxygen (by volume) in air. Below shows the set-up he used for the experiment. At the start of the experiment, gas syringe A contained 100 cm' of air and gas syringe B did not contain air. Peter passed the air over the hot copper powder by moving the plungers of the two syringes to and fro. He kept moving the plungers until there was no further change in the volume of gas in the set-up. Then he allowed the set-up to cool to room temperature, and then pushed all gases remained into gas syringe B. He found that 79 cm' of gas was collected in B. (a) The reddish brown copper powder became black during the experiment. Suggest why. (1 mark) (b) Suggest why the volume of gas trapped in the syringe decreased during the experiment. (1 mark) () Calculate the percentage of oxygen (by volume) in air. (2 marks)