Question

The vapour pressure of benzene at a certain temperature is 640 mm of Hg. A non-volatile and non-electrolyte solid weighing 2.175 g is added to 39.08 of benzene. If the vapour pressure of the solution is 600 mm of Hg, what is the molecular weight of solid substance ?

• A. 49.50
• B. 59.60
• C. 69.60
• D. 79.82

Answer 1

Answer: (C)

$\frac{P^{\circ }-P_s}{P^{\circ }}=\frac{\frac{w}{m}}{\frac{w}{m}+\frac{W}{M}}$
$\because \frac{W}{M}>\frac{w}{m}$
$\Rightarrow \frac{640-600}{640}$
$=\frac{w}{m}\times \frac{M}{W}$
$\Rightarrow \frac{40}{640}=\frac{2.175\times 78}{m\times 39.08},$
$m=\frac{2.175\times 78}{39.08}\times \frac{640}{40}$
$m=\mathrm{69.45.}$