Osmotic pressure observed when benzoic acid is dissolved in benzene is less than that expected from theoretical considerations. This is because:

Benzoic acid gets associated in benzene

When benzoic acid is dissolved in benzene, the observed osmotic pressure is less than what is expected based on theoretical considerations. This phenomenon occurs because benzoic acid molecules tend to associate with each other in benzene instead of behaving as individual solute particles.

Benzoic acid \(C_6H_5COOH\) is a carboxylic acid and consists of a benzene ring attached to a carboxyl group (-COOH). In its pure state, benzoic acid exists as individual molecules held together by intermolecular forces such as hydrogen bonding. However, when it is dissolved in benzene, which is a nonpolar solvent, the benzene molecules exert a unique influence on the behavior of benzoic acid.

In benzene, the nonpolar benzene molecules interact primarily through weak London dispersion forces. These forces are relatively weak compared to the hydrogen bonding forces present in benzoic acid. As a result, the benzene molecules have a tendency to form a more stable arrangement by interacting with each other through London dispersion forces rather than with the benzoic acid molecules.

When benzoic acid is added to benzene, the benzoic acid molecules try to minimize their interaction with the nonpolar benzene molecules. This leads to the formation of associated species or clusters of benzoic acid molecules within the benzene solvent. These associations can be in the form of dimers, where two benzoic acid molecules are held together by intermolecular hydrogen bonding or higher-order aggregates.

The association of benzoic acid molecules in benzene reduces the effective concentration of solute particles in the solution. Osmotic pressure is directly proportional to the concentration of solute particles. Hence, the observed osmotic pressure is lower than expected because the actual number of solute particles (associated benzoic acid clusters) is fewer than what would be predicted based on the concentration of benzoic acid.

It's worth noting that this phenomenon is specific to benzoic acid and certain other organic solutes that have the potential for molecular association in nonpolar solvents. In contrast, for substances that readily dissociate or do not form associations in the solvent, the observed osmotic pressure closely matches the theoretical predictions.

Therefore, the correct explanation for the observed lower osmotic pressure when benzoic acid is dissolved in benzene is that benzoic acid molecules tend to associate with each other in benzene, forming associated species or clusters and reducing the effective number of solute particles in the solution.