Acetic acid shows a molecular mass of 120 when dissolved in benzene. This is because of

Options

A. Solvation process
B. Intermolecular H-bonding
C. Presence of H atom at `alpha` C
D. Dimerisation

Answer

To solve the question regarding why acetic acid shows a molecular mass of 120 when dissolved in benzene, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Molecular Mass of Acetic Acid**:
- The molecular formula of acetic acid is \( CH_3COOH \).
- The molecular mass of acetic acid is calculated as follows:
- Carbon (C) = 12 g/mol, Hydrogen (H) = 1 g/mol, Oxygen (O) = 16 g/mol.
- Therefore, the molecular mass of acetic acid = \( 2 \times 12 + 4 \times 1 + 2 \times 16 = 60 \) g/mol.

2. **Dissolution in Benzene**:
- When acetic acid is dissolved in benzene, it does not remain as individual molecules. Instead, it tends to form interactions with other acetic acid molecules.

3. **Dimerization of Acetic Acid**:
- Acetic acid can undergo dimerization, where two molecules of acetic acid combine to form a dimer.
- This dimerization occurs due to the formation of hydrogen bonds between the hydroxyl group (-OH) of one acetic acid molecule and the carbonyl group (C=O) of another.

4. **Calculate the Molecular Mass of the Dimer**:
- When two acetic acid molecules dimerize, the effective molecular mass becomes:
- \( 60 \, \text{g/mol} \times 2 = 120 \, \text{g/mol} \).
- This explains why the observed molecular mass of acetic acid in benzene is 120 g/mol.

5. **Conclusion**:
- The reason for the observed molecular mass of 120 g/mol when acetic acid is dissolved in benzene is due to the dimerization of acetic acid molecules through intermolecular hydrogen bonding.

### Final Answer:
The molecular mass of acetic acid shows 120 g/mol when dissolved in benzene due to **dimerization** of acetic acid molecules.

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