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Question

Calculate the vapour pressure lowering caused by addition of 50 g of sucrose (molecular mass = 342) to 500 g water, if the vapour pressure of pure water at 25oC is 23.8 mm Hg.

A
0.94 mm Hg
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B
0.42 mm Hg
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C
1.24 mm Hg
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D
0.124 mm Hg
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Solution

The correct option is D 0.124 mm Hg
According to Raoult's law,
P0PsP0=nn+N
Where,
P0=Vapour pressure of waterPs=Vapour pressure of solutionn=number of moles of sucroseN=number of moles of water
ΔP=nn+NP0
Given: n=50342=0.146; N=50018=27.78 and p0=23.8
Substituting the values in the above equation,
ΔP=0.1460.146+27.78×23.8=0.124 mm Hg

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