If a solute ionizes into 3 particles in solution, how does it affect colligative properties relative to a nonelectrolyte?

• A. No effect
• B. It scales ΔTb and ΔTf by a factor of 3
• C. It multiplies ΔTb, ΔTf, and osmotic pressure by 3
• D. It decreases them by a factor of 3

Answer: (C)

The key idea is that colligative properties depend on the number of solute particles in solution, quantified by the van’t Hoff factor i. If the solute ionizes into three particles per formula unit, i = 3. Boiling point elevation and freezing point depression are both proportional to i (ΔTb = i Kb m and ΔTf = i Kf m), so they become three times larger. Osmotic pressure also scales with i (π = i M R T), so it is tripled as well. Thus, relative to a nonelectrolyte at the same concentration, all three properties increase by a factor of 3. (In reality, incomplete ionization or ion pairing could lower the effective i, but with complete ionization into three particles this is the outcome.)