When salt (non-volatile solute) is added to water kept in a sealed vessel, boiling point of the solution will be elevated or increased. So, vapour pressure of water (solution) will be decreased. Lowering of vapour pressure ( $\left.\Delta p=p^{\circ}-p\right)$ is also proportional to mole fraction of the solute $\left(\chi_{B}\right)$ present in the solution.
$\Delta p \propto \chi_{B}\left[p^{\circ}=\right.$ vapour pressure of pure water
$p=$ vapour pressure of solution $]$