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Vapour (2015)

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Addition of a non-volatile solute to a solvent lowers its vapour pressure. Therefore, the vapour pressure of a solution (i.e., V.P. of solvent in a solution) is lower than that of pure solvent, at the same temperature. A higher temperature is needed to raise the vapour pressure upto one atmosphere pressure, when boiling point is attained. However, increase in b.pt. is small. For example, \( 0.1 \) molal aqueous sucrose solution boils at \( 100.05^{\circ} \mathrm{C} \).

Sea water, an aqueous solution, which is rich in \( \mathrm{Na}^{+} \)and \( \mathrm{Cl}^{-} \)ions, freezes about \( 1^{\circ} \mathrm{C} \) lower than frozen water. At the freezing point of a pure solvent, the rates at which two molecules stick together to form the solid and leave it to return to liquid state are equal when solute is present. Few solvent molecules are in contact with surface of solid. However, the rate at which the solvent molecules leave the surface of solid remains unchanged. That is why, temperature is lowered to restore the equilibrium. The freezing point depression in a dilute solution is proportional to molality of the solute.
The mass of ice separated out on cooling a solution containing \( 50 \mathrm{~g} \) ethylene glycol in \( 200 \mathrm{~g} \) water to \( -9.3^{\circ} \mathrm{C} \) is : \( \left(K_{f}\right. \) for \( \mathrm{H}_{2} \mathrm{O}=1.86 \)
(A) \( 38.71 \mathrm{~g} \)
(B) \( 61.29 \mathrm{~g} \)
(C) \( 138.71 \mathrm{~g} \)
(D) \( 161.29 \mathrm{~g} \)
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