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Question

# The total vapour pressure of an ideal solution obtained by mixing 1 mol of 'A' and 2 mol of 'B' at 25∘C, is 180 torr. What is the vapour pressure (in torr) of pure 'B' at the same temperature? (Given, Vapour pressure of pure 'A' at 25∘C is 150 torr)

A
150
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B
125
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C
55
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D
195
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Solution

## The correct option is D 195Number of moles of A, nA=1 molNumber of moles of B, nB=2 mol Mole fraction of A =No. of moles of ATotal no. of moles of A and B xA=nAnA+nB=13xB=1−13=23By Raoult's law, pA=xA×poA pB=xB×poB where, poA is vapour pressure of pure A poB is vapour pressure of pure B Ptotal=xA×poA+xB×poB180=13×150+23×p∘B p∘B=195 torr

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