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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 0.6
p0= vapour pressure of pure solvent
ps= vapour pressure of of solvent in soultion

x2=mole fraction of solute in solution
x1=1-x2= mole fraction of solvent in solution

As we know that, (p0ps)/p0=x2
Case 1: Decrease in vapour pressure= 10mm of hg
10/ p0=0.2
p0=50 mm of hg

Case 2: Decrease in vapor pressure=20mm of hg
we find p0=50 mm of hg from case 1
20/50 = { x }_{ 2 }$
x2=0.4
x1=1-x2=0.6

So, the correct option is B

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