Question

Mixture(s) showing positive deviation from Raoult's law at ${35}^{\circ }C$ is (are)

• A. carbon tetrachloride + methanol
• B. carbon disulphide + acetone
• C. benzene + toluene
• D. phenol + aniline

Answer 1

Answer: (A), (B)

The correct options are
A carbon tetrachloride + methanol
B carbon disulphide + acetone

When intermolecular attraction between two components A and B in the mixture is same as between A and A, or B and B, it is a case of an ideal solution.

When intermolecular attraction between A and B in a mixture is smaller than that between A and A, or B and B, the mixture is more vaporised, b.p is lowered. It is a case of positive deviation from Raoult’s law.

When intermolecular attraction between A and B is higher than that between A and A, or B and B, the mixture is less vaporised, b.p is increased. It is a case of negative deviation.

(a) Methanol molecules $C{H}_{3}OH$ are hydrogen bonded. In a mixture of $CC{l}_{4}andC{H}_{3}OH$, the extent of H – bonding is decreased. The mixture is more vaporised thus, positive deviation from Raoult’s law.
(b) Acetone molecules have higher intermolecular attraction due to dipole-dipole interaction. With $C{S}_2$, this interaction is decreased thus, positive deviation.
(c) A mixture of benzene and toluene forms an ideal solution.
(d) Phenol and aniline have higher interaction due to intermolecular H – bonding. Hence, negative deviation.