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Question

The vapour pressure of a pure liquid A is 70 torr at 300 K. It forms an ideal solution with another liquid B. The mole fraction of B in the solution is 0.2 and total pressure of solution is 84 torr at 300 K. The vapour pressure of pure liquid B at 27C is ?

A
0.14 torr
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B
560 torr
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C
140 torr
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D
70 torr
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Solution

The correct option is C 140 torr
Solution:- (C) 140 torr.
Mole fraction of B=0.2

As we know that,
xA+xB=1
xA=10.2=0.8
pA=70 torr.
pB=?
ptotal=84 torr.

From Dalton's law of partial pressure,
ptotal=pA+pB.....(1)

From Raoult's law of vapour pressure,
pA=pAxA
pA=70×0.8=56

Again, from Raoult's law of vapour pressure,
pB=pBxB
pB=pB×0.2

Substituting these values in equation (1), we have
84=56+0.2×pB
pB=84560.2=140 torr.

Hence the vapour pressure of pure liquid B is 140 torr..

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