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 $ .
