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Question

# The vapour pressure of a solvent decreased by 10 mm of Hg when a non-volatile solute was added to the solvent. The mole fraction of solute in the solution is 0.2. What would be the mole fraction of solvent if decrease in vapour pressure is 20 mm?

A
0.8
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B
0.6
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C
0.4
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D
0.2
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Solution

## The correct option is C 0.6p0= vapour pressure of pure solventps= vapour pressure of of solvent in soultionx2=mole fraction of solute in solutionx1=1-x2= mole fraction of solvent in solutionAs we know that, (p0−ps)/p0=x2Case 1: Decrease in vapour pressure= 10mm of hg 10/ p0=0.2p0=50 mm of hgCase 2: Decrease in vapor pressure=20mm of hg we find p0=50 mm of hg from case 1 20/50 = { x }_{ 2 }\$x2=0.4x1=1-x2=0.6So, the correct option is B

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