Question

An aqueous solution is 1.00 molal in KI. Which change will cause the vapour pressure of the solution to increase?
(A) Addition of NaCl
(B) Addition of $N{{a}_{2}}S{{O}_{4}}$
(C) Addition of 1.00 molal KI
(D) Addition of water

Answer 1

Hint: The vapour pressure depends upon the surface area of the solution and is directly proportional to each other. Larger the surface area, higher will be vapour pressure. Addition of solute particles and solvent will result in different vapour pressures.

Complete answer:
Let us discuss the necessary essentials to solve the given illustration;
Vapour pressure-
It is a measure of tendency of a material to change itself into the gaseous or vapour phase.
Raoult’s law-
This law states that the partial vapour pressure of solvent in the solution is equal to the product of vapour pressure of the pure solvent and mole fraction of the solution.
Now, moving towards the conditions given in the above illustration;
If we add solute particles like NaCl, $N{{a}_{2}}S{{O}_{4}}$ or KI- they would act as electrolyte and will undergo ionisation which leads to lowering of vapour pressure. Whereas, addition of water will dilute the solution resulting in an increase in vapour pressure.

Therefore, option (D) is correct.

Note:
Do note that the addition of solute will decrease the vapour pressure as some of the sites of the surface will be occupied by the solute particles. Whereas, the addition of solvent will result in dilution, ultimately increasing the surface area of the liquid surface. Thus, increasing the vapour pressure.