Question

Which of the statements given below concerning describe a colligative effect ?

• A. Boiling point of pure water decreases by the addition of ethanol
• B. Vapour pressure of pure water decreases by the addition of nitric acid
• C. Vapour pressure of pure benzene decreases by the addition of naphthalene
• D. Boiling point of pure benzene increases by the addition of toluene

Answer 1

Answer: (C)

The relation, $\Delta T_b=k_b.m$ hold good only when solute is non-volatile. Toluene and ethanol are volatile solvents. Thus, choices (Boiling point of pure water decreases by the addition of ethanol) and (Boiling point of pure benzene increases by the addition of toluene) are wrong. Further, for a solute to show colligative effect in a solution, the solution should obey Raoult- law. A solution of $HN{O}_3$, in water shows negative deviation from Raoult- law. Thus, the only correct choice is (Vapour pressure of pure benzene decreases by the addition of naphthalene)