Question

When mercuric iodide is added to aqueous $KI$ solution:
(A) Freezing point is increased
(B) Osmotic pressure is raised
(C) Boiling point is elevated
(D) Vapour pressure is raised

Answer 1

To answer this question you should recall the concept of colligative properties. When mercury reacts with $KI$ , it causes a change in the number of particles i.e. colligative properties. Colligative properties are not dependent on the chemical nature of the solution’s components but their quantities present.

Complete step by step solution:
Colligative properties can be linked to several quantities that express the concentration of a solution, such as molarity, normality, and molality. The four colligative properties that can be exhibited by a solution are:
Boiling point elevation, Freezing point depression, Relative lowering of vapour pressure and Osmotic pressure.
When mercuric iodide is added to aqueous solution of Potassium iodide, the freezing point of solution will increase because Potassium iodide ionizes in water as $KI$ dissociates as:
$KI \to {K^ + } + {I^ - }$
When $Hg{I_2}\;$ is added to it, it forms a complex with $KI$ .
The reaction can be represented as: $2KI + Hg{I_2} \to {K_{2}}[Hg{I_4}]$ .
Hence, the number of particles will decrease from 4 to 3. So, there would be less depression in the freezing point and the freezing point would increase.
Thus, the correct option is A.

Note:
The Van’t Hoff factor which is an important aspect of colligative properties is defined as the ratio of the concentration of particles formed when a substance is dissolved to the concentration of the substance by mass. In the case of non-electrolytic substances which do not dissociate in water, the value of $i$ is generally 1. But in the case of ionic substances, due to dissociation, the value of $i$ is equal to the total number of ions present in one formula unit of the substance.