Question

An aqueous solution is $1$ molar in $K1$. Which change will cause the vapor pressure of the solution to increase?
(A) Addition of NaCl
(B) Addition of $N{a_2}S{O_4}$
(C) Addition of $1$ molar $K1$
(D) Addition of water

Answer 1

Vapor pressure depends upon the surface area of the solution. Larger the surface area, more will be the vapor pressure. Rather than this dependence of vapor pressure can also be explained with the help of volatile and non-volatile solutes.

Complete step by step answer: Vapor Pressure is a measure of the tendency of a material to change into the gaseous or vapor state.
Factors that affect the vapor pressure are as follows:
Nature of Liquid: It is explained on the basis of intermolecular forces. As the intermolecular forces will increase, vapor pressure decreases.

Temperature: As you know, the increase in temperature increases the kinetic energy which further increases the escaping tendency of molecules & hence, the vapor pressure increases.

Concentration of Solute: The Presence of solute also decreases the vapor pressure. So, we can say that the fall in vapor pressure varies with concentration of solute.
Vapor pressure increases with surface area, so when we add solute the surface area will indeed decrease causing a decrease in vapor pressure. Also, the concentration of solute will increase and that will also be responsible for the decrement.
However, when we add solvent, dilution increases the surface area of the liquid surface, thus this will result in increased vapor pressure.
Hence, the addition of water to the aqueous solution of $\left( {1\,\,{\text{molal}}} \right)$ KI, will result in an increase of vapor pressure.
So, the option D is correct.

Additional Information
Volatile substances are those that can evaporate or sublimate at room temperature or below that. These substances have higher vapor pressures than the non-volatile substances at the same temperature. Some of the examples of volatile substances are alcohol, mercury, gasoline. Water is less volatile than ethyl alcohol but is more volatile than oil.
Non-volatile substances are those that readily do not evaporate into a gas under existing conditions. These have low vapor pressure and a high boiling point. Sugar and salt are some examples of the non-volatile solutes.
Talking about the volatile substances, they have high vapor pressure but a low boiling point. Even if we increase the temperature, it will increase the vapor pressure respectively.
If we have two or more solutions, we can add them to get the total vapor pressure as shown below:
$P_{Total}$= $P_{Solution 1}$ + $P_{solution 2}$ +……….
The above equation represents that when we have two volatile solutions, they will push each other toward evaporation and have high vapor pressures and low boiling points. Whereas, when we have a non-volatile solution, such as a salt solution, we have an increasing boiling point as the concentration of salt increases. If we have water and a volatile substance, they will push further and further towards evaporation and higher vapor pressure will be there as the amount of volatile solution increases.
Hence, as ${\text{NaCl}}$ and ${\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ are non-volatile therefore, adding these two will decrease vapor pressure. So, addition of water will only lead to increase in the vapor pressure as water is a volatile substance.
Due to these reasons, option D is correct.

Note: Vapor Pressure is basically independent of volume of the container. As we know that liquid in the container will be in equilibrium with the vapor, the increase or decrease in volume will be due to the fact that some of the vapor gets converted into liquid or vice-versa.