Question

For a non-volatile solute:
(A) The vapour pressure of a solute is 0.
(B) Vapour pressure of solution = vapour pressure of solvent
(C) Vapour pressure of solution = vapour pressure of solvent in solution
(D) All of the above

Answer 1

Hint: A non-volatile solute is a substance which does not convert into vapour phase at normal conditions. They do not produce vapours and their boiling points are very high. When a non-volatile solute is dissolved in a solvent the vapour pressure of the solution decreases.

Complete step by step answer:
-First of all we will see what vapour pressure is.
Vapour pressure refers to the tendency of any substance to change into its vapour or gaseous phase with increase in temperature. Also the temperature at which the vapour pressure of the liquid surface becomes equal to the pressure exerted by the surroundings is known as the boiling point of a liquid.

-We will now see what a non-volatile substance and a volatile substance is.
A non-volatile solute basically refers to a substance which does not get evaporated to gas (vapour form) at room temperature. Such substances exhibit a low vapour pressure (almost negligible) and a high boiling point. Such solutes do not affect the vapour pressure of the solution and hence the vapour pressure of the non-volatile solutes in any solution is considered to be zero which means it does not produce any vapours.
A volatile substance is a substance which has the tendency to get converted into vapour form at the boiling points of the solutions. A volatile solute has higher vapour pressure as compared to the solvent and hence it produces vapours.

-We will now see how a non-volatile solute affects the vapour pressure.
When we add a non-volatile solute to a solvent, the vapour pressure of the solution decreases because the surface area is partly occupied by non-volatile solute molecules and the rest by the solvent molecules. This lowers the rate of evaporation.
-Hence from the above discussion we can say that the vapour pressure of the non-volatile solute is zero (0). So, the correct answer is “Option A”.

Note: Non-volatile substances have low vapour pressure and high boiling point. The vapour pressure of the solution is less than the vapour pressure of the solvent. Raoult’s law is applicable to solutions containing non-volatile solutes. According to the Raoult’s Law: The presence of the non-volatile solute lowers the vapour pressure of the solvent.
${P_{solution}} = {X_{solvent}}P_{solvent}^ \circ $
${P_{solution}}$ = Observed vapour pressure of the solution
${X_{solvent}}$ = Mole fraction of the solvent
$P_{solvent}^ \circ $ = Vapour pressure of pure solvent
Also the non-volatile solute increases the boiling point and the volatile solute decreases the boiling point of the solution. But all this occurs only if these solutes do not react within the solution.