Question

The vapour pressure of pure benzene at a certain temperature is 640 mm Hg. A non-volatile solid weighing 2.175 g is added to 39.0 g of benzene. The vapour pressure of the solution is 600 mm Hg. What is the molecular mass of the solid substance?

Answer 1

According to Raoult's law,
$\frac{P_0-P_s}{P_0}=\frac{n}{n+N}$
Let m be the molecular mass of the solid substance.
$n=\frac{2.175}{m};N=\frac{39}{78}=0.5$
(molecular mass of benzene = 78)
$P_0=640mm;P_s=600mm$
Substituting the values in above equation,
$\frac{640-600}{640}=\frac{\frac{2.175}{m}}{\frac{2.175}{m}+0.5}=\frac{2.175}{2.175+0.5m}$
$m=\frac{2.175\times 16-2.175}{0.5}=65.25$