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

Vapour pressure of pure 'A' is 70 mm of Hg at `25^(@)C`. It forms an ideal solution with 'B' in which mole fraction of A is 0.8 . If the vapour pressure of the solution is 84 mm of Hg at `25^(@)C`, 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
`p_("total")=p_(A)+p_(B)`
`=p_(A)^(@)x_(A)+p_(B)^(@)x_(B)`
`p_(A)^(@)=70` mm Hg, `x_(A)=0.8`
`p_(B)^(@)=? " " x_(B)=1-x_(A)=0.2`
`p_("total")=84` mm Hg.
`:.84=(70xx0.8)+(p_(B)^(@)xx0.2)`
`84=56+0.2 p_(B)^(@)`
`0.2p_(B)^(@)=84-56=28`
or `p_(B)^(@)=(28)/(0.2)=140` mm Hg.
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  • The vapour pressure of ethyl alcohol at 298 K is 40 mm of Hg. Its mole fraction in a solution with methyl alcohol is 0.80. Its vapour pressure in the solution if it obeys Raoult's law is :

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