Consider the following statements about abnormal molar masses and the van’t Hoff factor, i.
(A) When a solute dissociates into ions in solution, the experimentally determined molar mass is lower than the true value.
(B) If all the molecules of ethanoic acid associate into dimers in benzene, the depression in freezing point for ethanoic acid will be twice its normal value.
(C) For KCl, NaCl and MgSO4, the values of i approach 2 as the solution becomes very dilute.
(D) In case of association of the solute, the value of i is greater than unity.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (D) only
A colligative property depends on the number of solute particles. Dissociation increases this number and association decreases it, so the molar mass worked out from the property comes out abnormal.
(A) When a solute dissociates into ions in solution, the experimentally determined molar mass is lower than the true value. — Correct statement. More particles give a larger colligative effect, which is read as a smaller molar mass.
(B) If all the molecules of ethanoic acid associate into dimers in benzene, the depression in freezing point for ethanoic acid will be twice its normal value. — Incorrect statement. Dimerisation halves the number of particles, so ΔTf becomes half the normal value, and the molar mass worked out from it becomes twice the expected value.
(C) For KCl, NaCl and MgSO4, the values of i approach 2 as the solution becomes very dilute. — Correct statement. Each of these salts gives two ions, and on dilution the dissociation becomes nearly complete.
(D) In case of association of the solute, the value of i is greater than unity. — Incorrect statement. Association reduces the number of particles, so i is less than unity; i is greater than unity for dissociation.
NCERT (2026-27 edition):
1.7 Abnormal Molar Masses – “…This brings into light the rule that, when there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value.”
“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.”
“In case of association, value of i is less than unity while for dissociation it is greater than unity.”
“…For KCl, NaCl and MgSO4, i values approach 2 as the solution becomes very dilute.”