Question

State Raoult’s law for a solution containing volatile components. Why is the vapour pressure of an aqueous solution of glucose lower than that of water?

Hint:

Raoult’s law is particularly useful for ideal solutions, where the solute and solvent interactions are similar.

Answer 1

Raoult’s law states that the partial vapor pressure of each volatile component in a solution is directly proportional to its mole fraction in the solution. Mathematically, \[ P_A = X_A \cdot P_A^\circ \] where \( P_A \) is the partial vapor pressure of component A, \( X_A \) is its mole fraction, and \( P_A^\circ \) is the vapor pressure of the pure component A.
The vapor pressure of an aqueous solution of glucose is lower than that of water because glucose is a non-volatile solute. As the number of glucose molecules increases in the solution, they reduce the number of water molecules at the surface, thus lowering the vapor pressure. This is an example of the colligative property, where the vapor pressure depression is proportional to the amount of solute present.