At 300 K vapour pressure of a pure liquid. ' $A$ ' is 70 mm Hg . It forms an ideal solution with another liquid ' $B$ '. The mole fraction of $B$ in the solution is 0.2 and total vapour pressure of solution is 84 mm Hg at same temperature. What is the vapour pressure (in mm ) of pure liquid $B$ at 300 K ?

248 g of ethylene glycol $\left(\mathrm{C}_2 \mathrm{H}_6 \mathrm{O}_2\right)$ is added to 200 g of water to prepare antifreeze. What is the molality of resultant solution?

$$ (\mathrm{C}=12 \mathrm{u} ; \mathrm{H}=1 \mathrm{u} ; \mathrm{O}=16 \mathrm{u}) $$

A solution containing 7.5 g of urea (molar mass $=60 \mathrm{~g} \mathrm{~mol}^{-1}$ ) in 1 kg of water freezes at the same temperature as another solution containing 15 g of solute $X$, in the same amount of water. The molar mass of $X\left(\mathrm{~g} \mathrm{~mol}^{-1}\right)$ is