Osmotic pressure observed when benzoic acid is dissolved in benzene is less than that expected from theoretical considerations. This is because:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Benzoic acid gets associated in benzene
Osmotic pressure is a colligative property: it depends on the number of solute particles, \( \pi = iCRT \). In benzene, two benzoic acid molecules join through hydrogen bonding to form a dimer:
\( \displaystyle \ce{2C6H5COOH} \rightleftharpoons \ce{(C6H5COOH)2} \)
This association reduces the number of particles, so i is less than 1 and the observed osmotic pressure is lower than the value expected for single molecules.
Why the other options are incorrect
Option (1) Benzoic acid is an organic solute — Incorrect. A colligative property does not depend on the nature of the solute; an organic solute that stays as single molecules gives the expected value.
Option (2) Benzoic acid has higher molar mass than benzene — Incorrect. Osmotic pressure depends on the number of particles, not on how heavy each particle is.
Option (4) Benzoic acid gets dissociated in benzene — Incorrect. Dissociation would increase the number of particles (i > 1) and give a higher osmotic pressure, not a lower one.
NCERT (2026-27 edition):
1.7 Abnormal Molar Masses – “Molecules of ethanoic acid (acetic acid) dimerise in benzene due to hydrogen bonding. This normally happens in solvents of low dielectric constant. In this case the number of particles is reduced due to dimerisation.”
“… In case of association, value of i is less than unity while for dissociation it is greater than unity.”