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Question

The vapour pressure of pure benzene at a certain temperature is 640 mm Hg. A non-volatile solid weighing 2.175 g is added to 39.0 g of benzene. The vapour pressure of the solution is 600 mm Hg. What is the molecular mass of the solid substance?

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Solution

According to Raoult's law,
P0PsP0=nn+N
Let m be the molecular mass of the solid substance.
n=2.175m;N=3978=0.5
(molecular mass of benzene = 78)
P0=640mm;Ps=600mm
Substituting the values in above equation,
640600640=2.175m2.175m+0.5=2.1752.175+0.5m
m=2.175×162.1750.5=65.25

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