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Vapour pressure of benzene at 30^(@)C is...

Vapour pressure of benzene at `30^(@)C` is 121.8 mm. When 15 g of a non-volatile solute is dissolved in 250 g of benzene its vapour pressure decreased to 120.2 mm. The molecular weight of the solute is (mol. Weight of solvent = 78)

A

35.67 g

B

356.7 g

C

432.8 g

D

502.7 g

Text Solution

Verified by Experts

The correct Answer is:
B
Relative lowering of vapour pressure is given by
`(P^(0)-P)/(P^(0))=(w//m)/(w//m+W//M)`
where , `P^(0)` =Vapour pressure of pure solvent
P=Vapour pressure of solution
w=mass of slute
m=molecular mass of solute
W=mass of solvent
M=molecular mass of solvent.
For dilute solution
`implies (121.8-120.2)/(121.8)=(15//m)/(250//78)implies m=356.265`
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