Which of the statements giving below concerning properties of solutions, describes a colligative effect?

Question

 a) Boiling point of pure water decreases by the addition of ethanol

b) vapour presure 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  addition  of toluene 

 please also explain why  the options are incorrect. 

Answer 1

(c) vapour pressure of pure benzene decreases  by the addition  of naphthalene 

The colligative properties are properties of dilute solutions of non volatile solutes whose value depend on concentration of solute particles in the solution. Naphthalene is a non-volatile solid, hence in addition in benzene decrease its vapour pressure.