If all the molecules of ethanoic acid associate in benzene, then:

Question

Accepted Answer

When ethanoic acid is dissolved in benzene, it tends to associate, forming dimers due to hydrogen bonding. This association affects the colligative properties, such as the boiling point elevation ($\Delta T_{b}$).

1. **Explanation for Option 2 (Correct Answer):**  
   When ethanoic acid molecules associate to form dimers, the number of solute particles in the solution effectively decreases. Colligative properties, including boiling point elevation, depend on the number of solute particles. If all ethanoic acid molecules dimerize, the number of particles is halved compared to the situation where no association occurs. Consequently, the boiling point elevation ($\Delta T_{b}$) will be half of the normal value because the effect is directly proportional to the number of solute particles. Therefore, Option 2 is correct.

2. **Explanation for why other options are incorrect:**

- **Option 1:** This option suggests that $\Delta T_{b}$ will be double the normal value. This is incorrect because association reduces the number of solute particles, which decreases the boiling point elevation rather than increasing it.

- **Option 3:** This option implies that $\Delta T_{b}$ will have the normal value. This is incorrect because the association of molecules into dimers reduces the effective number of solute particles, thus reducing the boiling point elevation compared to the normal value.

- **Option 4:** This option states that the molar mass calculated based on $\Delta T_{b}$ will be half of the expected value. This is incorrect because the association leads to a higher apparent molar mass due to dimer formation. The calculated molar mass would be higher than expected, not lower.

In summary, Option 2 is correct because the association of ethanoic acid molecules into dimers reduces the number of solute particles, leading to a boiling point elevation that is half of the normal value.