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.
Which of the following aqueous solutions will show the maximum lowering of vapour pressure at 300 K?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1 M BaCl2
Lowering of vapour pressure is a colligative property: it depends on the number of solute particles, not on their nature. The passage states that ionic substances dissociate into ions, so their colligative properties are higher than those of non-electrolytes of the same concentration. All four solutions are 1 M, so the one that gives the most particles (largest van’t Hoff factor i) shows the greatest lowering.
\( \ce{BaCl2} \) — \( \ce{BaCl2 -> Ba^2+ + 2Cl^-} \): i = 3.
NaCl — \( \ce{NaCl -> Na^+ + Cl^-} \): i = 2.
Phenol — a very weak acid that ionises only slightly in water: i ≈ 1.
Sucrose — a non-electrolyte; it does not dissociate: i = 1.
\( \ce{BaCl2} \) gives the most particles, so its solution shows the maximum lowering of vapour pressure.