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Question

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

A
14
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B
56
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C
140
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D
70
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Solution

The correct option is B 140
We know Raoult's law,

For an ideal solution,

Partial pressure in solution = mole fraction × Vapour pressure in pure state

Given,

P0A=70 torr at 270C XB=0.2

then XA=1XB=10.2=0.8 P0B=?

Ptotal or Psolution =84 torr

By, Raoult's law

Psolution=XAP0A+XBP0B

84=0.8×70+0.2P0B

P0B=84560.2

P0B=140 torr

Vapour pressure of pure liquid B at 250C is 140 torr.

Hence, the correct option is C

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