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Vapour pressure of pure A is 70 mm of Hg...

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

A

56 mm

B

70 mm

C

140 mm

D

28 mm

Text Solution

Verified by Experts

The correct Answer is:
C
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`.
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