Vapour pressure of pure A is 70 mm of Hg at `25^(@)C`. If it forms an ideal solution with B in which mole fraction of A is 0.8 and vapour pressure of the solution is 84 mm of Hg at `25^(@)C`, then the vapour pressure of pure B at `25^(@)C` is

Options

A. 56 mm
B. 70 mm
C. 140 mm
D. 28 mm

Answer

C

Explanation

Given, `p_(A)^(o)=70mm`
`therefore x_(A)=0.8 therefore x_(B)=0.2`
and P = 84 mm
According to Raoult.s law,
`P=p_(A)^(o)x_(A)+p_(B)^(o)x_(B)implies84=70xx0.8+p_(B)^(o)xx0.2`
`implies p_(B)^(o)=(28)/(0.2)=140mm`.