According to the Raoult's law, vapour pressure of a solution is directly proportional to:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Mole fraction
Raoult’s law states that, for a solution of volatile liquids, the partial vapour pressure of each component is directly proportional to its mole fraction in the solution: pi = xi pi0. For a solution of a non-volatile solute, the vapour pressure of the solution is proportional to the mole fraction of the solvent.
Why the other options are incorrect
Option (2) Parts per million — Incorrect. A unit for very dilute solutions; it does not appear in Raoult’s law.
Option (3) Mass percentage — Incorrect. Vapour pressure depends on the relative number of molecules, not on their mass.
Option (4) Molality — Incorrect. Molality appears in the expressions for elevation of boiling point and depression of freezing point, not in Raoult’s law.
NCERT (2026-27 edition):
Vapour Pressure of Liquid-Liquid Solutions – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 | – |