Solutions Question 125

Question: The vapour pressure of benzene at a certain temperature is $ 640,mm $ of $ Hg $ . A non-volatile and non-electrolyte solid weighing $ 2.175g $ is added to $ 39.08g $ of benzene. The vapour pressure of the solution is $ 600mm $ of $ Hg $ . What is the molecular weight of solid substance [CBSE PMT 1999; AFMC 1999]

Options:

A) 49.50

B) 59.6

C) 69.5

D) 79.8

Show Answer

Answer:

Correct Answer: C

Solution:

$ \frac{P^{0}-P_{s}}{P^{0}}=\frac{\frac{w}{m}}{\frac{w}{m}+\frac{W}{M}} $

$ \because ,\frac{W}{M}>\frac{w}{m} $
$ \Rightarrow \frac{640-600}{640} $

$ =\frac{w}{m}\times \frac{M}{W}\Rightarrow \frac{40}{640}=\frac{2.175\times 78}{m\times 39.08} $ ; $ m=\frac{2.175\times 78}{39.08}\times \frac{640}{40} $

$ m=69.45 $ .



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