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

Answer

a

Explanation

`p = p_(A)^(@) X_(A) + p_(B)^(@) X_(B)`
`hArr 84 = 70xx0.8 + p_(B)^(@) xx 0.2`
`p_(B)^(@) = (28)/(0.2) = 140 mm`