Statement I: The relative lowering of vapour pressure of a solution is independent of temperature.
Statement II: The relative lowering of vapour pressure is equal to the mole fraction of the solute.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I
The relative lowering of vapour pressure is the lowering of the vapour pressure of the solvent divided by the vapour pressure of the pure solvent. By Raoult’s law, for a solution of a non-volatile solute:
\( \displaystyle \frac{p_1^0-p_1}{p_1^0}=x_2 \)
Statement I: The relative lowering of vapour pressure of a solution is independent of temperature. — True. The vapour pressures p10 and p1 both change with temperature, but their ratio stays equal to x2, and a mole fraction does not depend on temperature.
Statement II: The relative lowering of vapour pressure is equal to the mole fraction of the solute. — True. This is Raoult’s law for a non-volatile solute, shown in the equation above.
Why Statement II explains Statement I?
A mole fraction is a ratio of numbers of moles, and the number of moles of each component does not change when the solution is heated or cooled.
Since the relative lowering is equal to this mole fraction (Statement II), it too stays the same at every temperature (Statement I).
NCERT (2026-27 edition):
1.6.1 Relative Lowering of Vapour Pressure – “…
\( \displaystyle \frac{\Delta p_1}{p_1^0}=\frac{p_1^0-p_1}{p_1^0}=x_2 \qquad (1.25) \)
The expression on the left hand side of the equation as mentioned earlier is called relative lowering of vapour pressure and is equal to the mole fraction of the solute.”
1.2 Expressing Concentration of Solutions – “Each method of expressing concentration of the solutions has its own merits and demerits. Mass %, ppm, mole fraction and molality are independent of temperature, whereas molarity is a function of temperature. This is because volume depends on temperature and the mass does not.”