Question

The vapour pressure of pure benzene at a certain temperature is $640mmHg$. A non-volatile solid weighing $2.715g$ is added to $39.0g$ of benzene.

The vapour pressure of the solution is $600mmHg$. What is the molar mass of the solid substance?

Answer 1

Given that,
$P^{\circ }=640mm,P_s=600mm,w=2.175g,W=39.0g,M=78$
$\because \frac{P^0-P_s}{P_s}=\frac{w\times M}{m\times W}$
$\therefore \frac{640-600}{600}=\frac{2.175\times 78}{m\times W}$
$\therefore m=65.25$
Molar mass = $69.6g/mole$