Which one of the following given below concerning properties of solutions, describe a colligative effect ?

Options

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 napthalene
D. Boiling point of pure benzene increases by the addition of toluene

Answer

C

Explanation

The relation, `DeltaT_(b) = k_(b).m` hold good only when solute is non-volatile. Toulene and ethanol are volatile solvents. Thus, choices (A) and (D) are wrong. Further, for a solute to show colligative effect in a solution, the solution should obey Raoult's law. A solutions of `HNO_(3)`, in water shows negative deviation from Raoults' law. Thus, the only correct choice is (C).