The vapour pressure of a pure liquid 'A' is 70 torr at `27^(@)C`. It forms an ideal solution with another liquid B. The mole fraction of B is 0.2 and total pressure of the solution is 84 torr at `27^(@)C`. The vapour pressure of pure liquid B at `27^(@)C` is :

Options

A. 14 torr
B. 56 torr
C. 140 torr
D. 70 torr

Answer

C

Explanation

Mole of fraction of A = 1- 0.2 = 0.8
`p_(A)^(@) xx x_(A) + p_(B)^(@) xx x_(B) = p`
`P_(A)^(@) = 70 "torr" , p_(B)^(@) = ? p = 84 ` torr
`70 xx 0.8 + p_(B)^(@) xx 0.2 = 84`
`p_(B)^(@) xx 0.2 = 84 - 56 = 28 ` torr
`p_(B)^(@) = (28)/(0.2) = 140` torr