The vapour pressure of pure liquids $$A$$ and $$B$$ are 450 and $$700 \mathrm{~mm}$$ of $$\mathrm{Hg}$$ at $$350 \mathrm{~K}$$ respectively. If the total vapour pressure of the mixture is $$600 \mathrm{~mm}$$ of $$\mathrm{Hg}$$, the composition of the mixture in the solution is

Which of the following pair of solutions is isotonic?

Solute '$$X$$' dimerises in water to the extent of $$80\%$$. 2.5g of '$$X$$' in $$100 \mathrm{~g}$$ of water increases the boiling point by $$0.3^{\circ} \mathrm{C}$$. The molar mass of 'X' is $$[K_b=0.52 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}]$$

A non-volatile solute, '$$A$$' tetramerises in water to the extent of $$80 \%.$$ $$2 .5 \mathrm{~g}$$ of '$$A$$' in $$100 \mathrm{~g}$$ of water, lower the freezing point by $$0.3^{\circ} \mathrm{C}$$. The molar mass of '$$A$$' in $$\mathrm{g}$$ is $$\left(K_f\right.$$ for water $$=1.86 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1})$$