Question

The molecular mass of acetic acid dissolved in water is \(60\) and when dissolved in benzene it is \(120\). This difference in behaviour of \(CH_3COOH\) is because

• A. Water prevents association of acetic acid
• B. Acetic acid does not fully dissolve in water
• C. Acetic acid fully dissolves in benzene
• D. Acetic acid does not ionize in benzene

Hint:

Carboxylic acids often form dimers in non-polar solvents like benzene, giving abnormally high molecular mass.

Answer 1

The Correct Option is A

Concept:
Acetic acid has normal molecular mass: \[ CH_3COOH = 60 \] In non-polar solvents like benzene, acetic acid molecules associate with each other through hydrogen bonding and form dimers. A dimer means two molecules join together: \[ 2CH_3COOH \rightarrow (CH_3COOH)_2 \] So the observed molecular mass becomes: \[ 2\times60=120 \]

Step 1: Understand behaviour in water.
Water is a polar solvent. Acetic acid forms hydrogen bonds with water molecules. Because water surrounds and solvates acetic acid molecules, acetic acid molecules do not associate strongly with each other. Therefore, molecular mass remains close to its normal value: \[ 60 \]

Step 2: Understand behaviour in benzene.
Benzene is a non-polar solvent. In benzene, acetic acid molecules are not strongly solvated by the solvent. So acetic acid molecules associate with each other through intermolecular hydrogen bonding. This leads to dimer formation: \[ (CH_3COOH)_2 \] Observed molecular mass: \[ 120 \]

Step 3: Reason for difference.
In water: \[ \text{association is prevented} \] In benzene: \[ \text{association occurs} \] Therefore, the difference in molecular mass is due to the fact that water prevents association of acetic acid.

Step 4: Check the options.
Option (A) is correct because water prevents association by hydrogen bonding with acetic acid.
Option (B) is incorrect because acetic acid dissolves in water.
Option (C) is not the reason for molecular mass becoming \(120\).
Option (D) is true in general, but it does not explain the association difference as directly as option (A). Hence, the correct answer is: \[ \boxed{(A)\ \text{Water prevents association of acetic acid}} \]