Question

Calculate the vapour pressure lowering caused by addition of 50 g of sucrose (molecular mass = 342) to 500 g of water if the vapour pressure of pure water at ${25}^{o}C$ is 23.8 mm Hg.

Answer 1

According to Raoult's law,

$\frac{P_0-P_s}{P_0}=\frac{n}{n+N}$

or $△p=\frac{n}{n+N}.P_0$

Given: $n=\frac{50}{342}=0.146;N=\frac{500}{18}=27.78$ and $P_0=23.8mmHg$

Substituting the value in the above equation,

$△p=\frac{0.146}{0.146+27.78}\times 23.8=0.124mmHg$