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

Answer

C

Explanation

`"Given, "p_(A)^(@)=70mm" "(becausex_(A)=0.8) `
`:.x_(B)=0.2andp=84mm`
According to Raoult's law, `p=p_(A)^(@)x_(A)+p_(B)^(@)x_(B)^(@)`
`84=0.8xx70+0.2xxp_(B)^(@)`
`p_(B)^(@)=(28)/(0.2)=140mm`