Chemistry - Liquid Solution Question with Solution | TestHub

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 $$\Rightarrow \frac{121.8-120.2}{121.8}=\frac{15/m}{250/78}\Rightarrow m=356.265$$