The vapour pressure of pure benzene at $25^{\large\circ}C$ is 639.7mm of mercury and the vapour pressure of a solution of a solute in benzene at the same temperature is $631.9$mm of mercury.Calculate the molality of the solution.

Question

$\begin{array}{1 1}0.156mole/kg\\0.146mole/kg\\0.126mole/kg\\0.176mole/kg\end{array} $