Assertion (A): The molar mass of ethanoic acid determined from the depression in freezing point of its solution in benzene is higher than its normal molar mass.
Reason (R): Ethanoic acid molecules dimerise in benzene because benzene is a solvent of high dielectric constant.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
A colligative property depends on the number of solute particles. When molecules of a solute associate, the number of particles falls, and the molar mass worked out from the property comes out abnormal.
\( \displaystyle \ce{2CH3COOH <=> (CH3COOH)2} \)
Assertion (A): The molar mass of ethanoic acid determined from the depression in freezing point of its solution in benzene is higher than its normal molar mass. — True. Dimerisation lowers the number of particles, so ΔTf is smaller than expected. A smaller ΔTf gives a larger molar mass; if all the molecules dimerise, it comes out twice the normal value.
Reason (R): Ethanoic acid molecules dimerise in benzene because benzene is a solvent of high dielectric constant. — False. The dimers are held by hydrogen bonding, and such dimerisation normally happens in solvents of low dielectric constant, such as benzene.
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. … It can be undoubtedly stated here that if all the molecules of ethanoic acid associate in benzene, then ΔTb or ΔTf for ethanoic acid will be half of the normal value. The molar mass calculated on the basis of this ΔTb or ΔTf will, therefore, be twice the expected value.”