Question

The vapour pressure of pure benzene is $639.70\, mm$ of $Hg$ and the vapour pressure of solution of a solute in benzene at the same temperature is $631.9 \,mm$ of $Hg$. Calculate the molality of the solution.

Answer 1

According to Raoult's law :
$p=p_{0} \chi_{1} \Rightarrow 631.9=639.7 \chi_{1}$
$\Rightarrow \chi_{1}=0.9878 \Rightarrow \chi_{2}=0.0122$
$\Rightarrow $ Molality $=\frac{0.0122}{0.9878 \times 78} \times 1000=0.158$