Question

The vapour pressure of a solution of non-volatile solute is (a) less than that of solvent (b) equal to that of solvent (c) more than that of solvent (d) equal to or more than that of solvent

Short answer

The vapour pressure of a solution of non-volatile solute is (a) less than that of solvent.

Step-by-step solution

Understand Vapor Pressure of Pure Solvent

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase at a given temperature in a closed system. For a pure solvent, the vapor pressure is at its maximum value since there are no other substances present in the liquid to disrupt the escape of molecules from the liquid phase to the vapor phase.

Comparing with Solution of Non-Volatile Solute

When a non-volatile solute is added to the solvent, it reduces the number of solvent molecules available at the surface to escape into the vapor phase. This leads to a decrease in the rate of vaporization and thus the vapor pressure of the solution becomes less than that of the pure solvent.

Understanding Raoult's Law

Raoult's Law states that the vapor pressure of an ideal solution is directly proportional to the mole fraction of solvent present in the solution. Hence, the presence of a non-volatile solute lowers the vapor pressure of the solution because the mole fraction of the solvent decreases.

Key concepts

Raoult's Law

Understanding the principles governing the behavior of solutions can sometimes be challenging, but Raoult's Law offers a simplified and predictive approach. It tells us how the vapor pressure of a solution changes when a solute is added to a solvent. This law states that the vapor pressure of the solvent in an ideal solution is directly proportional to the mole fraction of the solvent in the mixture.

Raoult's Law can be mathematically expressed as: \( P = P^\ast \cdot X_{solvent} \) where \( P \) is the vapor pressure of the solution, \( P^\ast \) is the vapor pressure of the pure solvent, and \( X_{solvent} \) is the mole fraction of the solvent. In an ideal solution where intermolecular forces are similar, adding a non-volatile solute—which does not contribute to the vapor pressure—reduces the mole fraction of the solvent, and therefore, reduces the overall vapor pressure of the solution in accordance with Raoult's Law.

Non-volatile Solute

In the context of vapor pressure, a non-volatile solute plays a significant role. By definition, non-volatile substances have little to no tendency to vaporize; that is, they exhibit negligible vapor pressure at a given temperature. When such a solute is dissolved in a solvent, it affects the overall vapor pressure of the resulting solution.

Here's why: the solute particles essentially 'take up space' on the surface of the liquid and in the solution, thus hindering the escape of solvent molecules into the vapor phase. This blockade decreases the number of solvent molecules that are able to vaporize, thereby reducing the vaporization rate. Additionally, because the solute does not add to the vapor pressure, the overall pressure of the solution will be lower than that of the pure solvent. Understanding this concept is crucial for students as it underlies many processes in chemistry, from colligative properties to real-world applications like antifreeze in car engines.

Vaporization Rate

The rate of vaporization is a measure of how quickly molecules of a liquid transition into the vapor phase. In a pure solvent, this rate is governed by factors such as temperature, the nature of the solvent, and the surface area. At a given temperature, molecules within the liquid have varying kinetic energies, and when these molecules gain enough energy to overcome intermolecular forces holding them together, they can escape into the vapor phase.

When a non-volatile solute is introduced, it interferes with the path of the solvent molecules attempting to evaporate, effectively putting a 'brake' on their vaporization rate. This leads to a lower concentration of solvent molecules in the vapor phase above the solution, equating to a lower vapor pressure. Remember, vapor pressure is directly related to the number of molecules that have transitioned into the gaseous state; fewer molecules evaporating means lower vapor pressure. This insight is not just a textbook concept; it's crucial in fields like meteorology and culinary arts, where understanding evaporation is integral to weather prediction and cooking techniques respectively.