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Question

Estimate the lowering of vapour pressure due to the solute (glucose) in a 1.0 M aqueous solution at 100oC.

A
10 torr
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B
18 torr
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C
13.45 torr
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D
24 torr
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Solution

The correct option is C 13.45 torr
First, we have to calculate the mole fraction of gas.
Let us assume that mole fraction of gas is x before as we know, the relation between mole fraction and

Molality Molarity =x×1000(1x)M

where M is the molecular weight of a solvent for aqueous solution, the solvent is water.
The molecular weight of water, M=18 g/mol

Now,
1=x×1000(1x)×18

18(1x)=1000x

18=(1000+18)x

x=181018

Now use formula
Relative lowering of VP = mole fraction gas

Δpp0=x

here p0 is initial pressure, at STP
p0=760 mmHg

So,
Δp=760×181018=13.44 torr approx

Hence 13.45torr

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