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 is based on the addition of the solute in the solution. When a non-volatile solute is added to the solution, then the mole fraction will also increase, this will decrease the vapour pressure of the solution.

Complete step by step solution:
The pressure exerted by the vapours in equilibrium with the liquid/solution at a particular pressure is known as the vapour pressure of a liquid/solution.
As the intermolecular forces decrease, the vapour pressure increases because more and more molecules can break the bonds in the liquid state and come into the vapour state. The vapour pressure also increases when the temperature increases, because of an increase in the kinetic energy of the molecules.
So, according to Raoult’s Law for Non-Volatile Solutes: When a non-volatile solute is added to the solution, then the mole fraction will also increase, this will decrease the vapour pressure of the solution.
This means that when the electrolytes are added to a solution, the vapour pressure of the solution decreases.
So, the $NaCl$, $N{{a}_{2}}S{{O}_{4}}$, and $KI$ are electrolytes and when they are dissolved in the solution, it will dissociate into ions and decrease the vapour pressure. But water is not an electrolyte, so it will increase the vapour pressure.

Therefore, the correct answer is an option (D)- Addition of water.

Note: The vapour pressure of a component at a given temperature in the solution, can be calculated by multiplying the mole fraction of the component to the vapour pressure of the component in the pure state.