1.

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 :

Answer»

56 mm
70 mm
140 mm
28 mm

Solution :`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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