Question

A solution is prepared by dissolving 10 g of non-volatile solute in 200 g of water. It has a vapour pressure of 31.84 mm Hg at 308 K. Calculate the molar mass of the solute.
(Vapour pressure of pure water at 308 K = 32 mm Hg).

Answer 1

According to raoult's law,

$\frac{p^0-p}{p^0}=\frac{w\times M}{m\times W}$

Where,

$p^0$ is Vapour pressure of pure water

$p$ is vapour pressure of solvent

$w$ is weight of solute

$m$ is molecular weight of solute

$W$ is molecular weight of solvent

$\frac{32-31.84}{32}=\frac{10\times 18}{m\times 200}$

$\Rightarrow m=\frac{0.9}{0.005}=180$