Liquid Solution

At $50^{\circ} \mathrm{C}$, the vapour pressure of pure benzene is 268 torr. The number of moles of non-volatile solute per mole of benzene required to prepare a solution having a vapour pressure of 167 torr at the same temperature is (molar mass of benzene $=78 \mathrm{~g} \mathrm{~mol}^{-1}$ )