Question

Which of the following liquid pairs shows a positive deviation from Raoult's law?

• A. Water - hydrochloric acid
• B. Water - nitric acid
• C. Acetone - chloroform
• D. Benzene - methanol

Answer 1

Answer: (D)

In solutions showing positive deviation, the observed vapour pressure of each component and total vapour pressure are greater than predicted by Raoult's law, i.e.
${\text{P}}_{\text{Α}}>\text{P}{}_{\text{Α}}^{{}^{\circ }}{\text{x}}_{\text{Α}}\text{;}\text{P}{}_{\text{Β}}>\text{P}{}_{\text{Β}}^{{}^{\circ }}{\text{x}}_{\text{Β}}\text{;}\text{P}>\text{P}{}_{\text{Α}}+\text{P}{}_{\text{Β}}$
In solution of methanol and benzene , methanol molecules are held together due to a hydrogen bonding as shown below:

On adding benzene, the benzene molecules get in between the molecules of methanol, thus breaking the hydrogen bonds. As the resulting solution has weaker intermolecular attractions, the escaping tendency of alcohol and benzene molecules from the solution increases. Consequently, the vapour pressure of the solution is greater than the vapour pressure as expected from Raoult's law.