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Question

The weight of non-volatile solute having molecular weight 40, that should be dissolved in 43 g hexane to reduce its vapour pressure to 80% is:

A
5 g
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B
47.2 g
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C
95 g
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D
80 g
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Solution

The correct option is A 5 g
Solution:- (A) 5g
Let the vapour pressure of pure octane be p.
Then, the vapour pressure of the octane after dissolving the non-volatile solute (ps) is-
ps=80100p=0.8p
Molar mass of solute (M2)=40g/mol
Mass of hexane (w1)=43g
Molar mass of hexane (M1)=86g
From Rault's law,
ppsps=n1n2
ppsps=(w2M2)(w1M1)
p0.8p0.8p=(w240)(4386)
w2=0.2×200.8=5g
Hence the weight of non-volatile solute reqired is 5gm.

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