Vapour pressure of benzene at 30^ C is 121.8 ~mm . When 15 ~g of a...

Relative lowering of vapour pressure is given by : $\frac{P^0-P}{P^0}=\frac{w / m}{w / m+W / M}$Where, $P^0=$ Vapour pressure of pure solvent $P=$ Vapour pressure of solution $w$ mass of solute $m=$ molecular mass of solute $W$ = mass of solvent $M=$ molecular mass of solvent. For dilute solution \Rightarrow \quad $\frac{121.8-120.2}{121.8}=\frac{15 / m}{250 / 78} \Rightarrow$m=356.265