19. Vapour pressure of pure ‘A’ is 70 mm of Hg at

25° C. It forms an 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 25° C , the vapour pressure of pure ‘B’ at 25° C is

a. 56 mm

b. 70 mm

c. 140 mm

d. 28 mm

Ans. c