Intermolecular hydrogen bonding is observed between chloroform and acetone. Therefore, their binary solution will _____.
(A) Show positive deviation from Raoult's law
(B) Show negative deviation from Raoult's law
(C) Obey Raoult's law
(D) Show decrease in vapor pressure
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (D) are correct
A solution obeys Raoult’s law (an ideal solution) when the attraction between unlike molecules A–B is about the same as that between like molecules A–A and B–B. If A–B attractions are stronger, the molecules escape less easily, the vapour pressure is lower than Raoult’s law predicts, and the solution shows a negative deviation. If A–B attractions are weaker, the vapour pressure is higher (positive deviation).
The hydrogen of chloroform is made slightly acidic by the three chlorine atoms, and it forms a hydrogen bond with the oxygen of acetone, which neither liquid has on its own:
\( \ce{Cl3C-H} \) ··· \( \ce{O=C(CH3)2} \)
(A) Show positive deviation from Raoult’s law — Incorrect. Positive deviation needs weaker A–B attractions, as in ethanol and acetone, where mixing breaks hydrogen bonds.
(B) Show negative deviation from Raoult’s law — Correct. The new hydrogen bonds make A–B attractions stronger than A–A and B–B.
(C) Obey Raoult’s law — Incorrect. An ideal solution needs equal attractions; here they are unequal.
(D) Show decrease in vapor pressure — Correct. The escaping tendency of each component falls, so the vapour pressure is lower than the ideal value.
NCERT (2026-27 edition):
Non-ideal Solutions – “… In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline. In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules. Similarly, a mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law.”