Which quantity is represented by the relative lowering in vapour pressure of an ideal solution containing the non-volatile solute?

Mole fraction of the solute

The relative lowering in vapor pressure of an ideal solution containing a non-volatile solute is represented by the mole fraction of the solute $(X_2)$.

The relative lowering in vapor pressure ($\Delta P/P_0$) of an ideal solution is given by Raoult's law:

\[\frac{\Delta P}{P_0} = X_2\]

where $\Delta P$ is the lowering in vapor pressure, $P_0$ is the vapor pressure of the pure solvent, and $X_2$ is the mole fraction of the solute.

Raoult's law states that the vapor pressure of a component in an ideal solution is directly proportional to its mole fraction in the solution. In the case of a non-volatile solute, the vapor pressure of the solvent is lowered, and the extent of this lowering is determined by the mole fraction of the solute.

Therefore, the mole fraction of the solute ($X_2$) is the quantity that represents the relative lowering in vapor pressure of an ideal solution containing a non-volatile solute.