The molar masses determined from the colligative properties of substances that associate or dissociate in solution are abnormal and are called abnormal molar masses. Ionic substances such as NaCl, BaCl2 and AlCl3 dissociate into ions in solution. Their colligative properties are higher than those of solutions of non-electrolytes such as glucose of the same concentration, because the number of particles in solution increases. When molecules of a substance such as ethanoic acid (acetic acid) associate in benzene to form dimers, the number of particles decreases and the colligative properties also decrease. The van’t Hoff factor, i, accounts for the extent of dissociation or association:
\(\displaystyle i = \frac{\text{Normal molar mass}}{\text{Abnormal molar mass}} = \frac{\text{Observed colligative property}}{\text{Calculated colligative property}}\)
\(\displaystyle i = \frac{\text{Total number of moles of particles after association/dissociation}}{\text{Number of moles of particles before association/dissociation}}\)
The value of i is greater than 1 for ionic substances and less than 1 for substances that associate in solution.
Consider the following statements about solutes that associate or dissociate in solution.
(A) Molecules of ethanoic acid dimerise in benzene due to hydrogen bonding.
(B) Dimerisation of ethanoic acid normally happens in solvents of high dielectric constant.
(C) For a solute with van’t Hoff factor i, the osmotic pressure of its solution is given by Π = i n2RT/V.
(D) Calculated colligative properties are obtained by assuming that the non-volatile solute dissociates into ions.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
The van’t Hoff factor compares the observed colligative property with the value calculated for a solute that neither associates nor dissociates. The colligative property equations are corrected by multiplying by i.
(A) Molecules of ethanoic acid dimerise in benzene due to hydrogen bonding. — True. Two molecules join through hydrogen bonds to form a dimer.
\( \displaystyle \ce{2CH3COOH <=> (CH3COOH)2} \)
(B) Dimerisation of ethanoic acid normally happens in solvents of high dielectric constant. — False. It normally happens in solvents of low dielectric constant, such as benzene.
(C) For a solute with van’t Hoff factor i, the osmotic pressure of its solution is given by Π = i n2RT/V. — True. This is the modified equation for osmotic pressure.
(D) Calculated colligative properties are obtained by assuming that the non-volatile solute dissociates into ions. — False. They are calculated assuming the solute is neither associated nor dissociated; that is why i = observed / calculated tells us the extent of association or dissociation.
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.”
“Here abnormal molar mass is the experimentally determined molar mass and calculated colligative properties are obtained by assuming that the non-volatile solute is neither associated nor dissociated.”
“Inclusion of van’t Hoff factor modifies the equations for colligative properties as follows: … Osmotic pressure of solution, Π = i n2 R T / V”