Solutions and Colligative Properties 1 Question 14
15. Mixture(s) showing positive deviation from Raoult’s law at $35^{\circ} \mathrm{C}$ is (are)
(2016 Adv.)
(a) carbon tetrachloride + methanol
(b) carbon disulphide + acetone
(c) benzene + toluene
(d) phenol + aniline
Numerical Value Based Question
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Solution:
- When intermolecular attraction between two components $A$ and $B$ in the mixture is same as between $A$ and $A$ or $B$ and $B$, hence it is a case of ideal solution.
When intermolecular attraction between $A$ and $B$ in a mixture is smaller than that between $A$ and $A$ or $B$ and $B$, then mixture is more vaporised, $\mathrm{bp}$ 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 $B$, then mixture is less vaporised, $\mathrm{bp}$ is increased. It is a case of negative deviation.
(a) Methanol molecules $\left(\mathrm{CH}{3} \mathrm{OH}\right)$ are hydrogen bonded. In a mixture of $\mathrm{CCl}{4}$ and $\mathrm{CH}_{3} \mathrm{OH}$, extent of $\mathrm{H}$-bonding is decreased. Mixture is more vaporised thus, positive deviation from Raoult’s law.
(b) Acetone molecules have higher intermolecular attraction due to dipole-dipole interaction. With $\mathrm{CS}_{2}$, this interaction is decreased thus, positive deviation.
(c) Mixture of benzene and toluene forms ideal solution.
(d) Phenol and aniline have higher interaction due to intermolecular $\mathrm{H}$-bonding. Hence, negative deviation.
