Vapour pressure of pure ‘A’ is 70 mm of Hg at 25ºC. It forms and ideal solution with ‘B’ in which mole fraction of A is 0.8. If the vapour pressure of the solution is 84 mm of Hg at 25ºC, the vapour pressure of pure ‘B’ at 25ºC is

Given: Vapour pressure of a pure component A and B are as follows, 0 A A B P = 70 mm of Hg X 0.8 X 1 0.8 0.2 The total vapour pressure of the solution is written as, 0 0 Total A A B B P = P X P X Substitute values in above equation. We have, 0 B 0 B 0 B 84 = 70 0.8 P 1 0.8 0.2P = 84 56 28 P 140 mm of Hg 0.2 x >> >> Thus, (c) is the correct option.