Paper 2 · Diffusion of ammonia and hydrogen chloride; methods of purification
Cotton wool soaked in concentrated ammonia is placed at one end of a glass tube, and cotton wool soaked in concentrated hydrochloric acid is placed at the other end. White fumes of ammonium chloride form off-centre in the tube, closer to the hydrochloric acid end. This happens because...
Ahydrogen chloride has a smaller molar mass than ammonia, so it diffuses more quickly; the fumes therefore form closer to the hydrochloric acid end.
Bhydrogen chloride has a larger molar mass than ammonia, so it diffuses more slowly; the fumes therefore form closer to the hydrochloric acid end.
Cammonia reacts with the glass tube before it can reach the centre, so the fumes form nearer the hydrochloric acid end.
Dhydrogen chloride is denser than air and sinks within the tube, so the fumes form nearer the hydrochloric acid end.
Explanation: HCl (Mr = 36.5) has a larger molar mass than NH3 (Mr = 17), so it diffuses more slowly and travels a shorter distance in the same time, meaning the two gases meet closer to the hydrochloric acid end.
Derived from ZIMSEC Chemistry 5071/2 Paper 2 Theory, November 2017, Q1
