Which one of the following 1 molal aqueous solution will show maximum freezing point depression?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → [Co(H2O)6]Cl3
Depression of freezing point is a colligative property: \( \Delta T_f = i\,K_f\,m \). All the solutions are 1 molal in the same solvent, so the one giving the most particles (largest i) shows the largest depression. In a coordination compound the complex ion stays intact; only the ions outside the square bracket separate, and water of crystallisation gives no ions:
[Co(H2O)3Cl3]·3H2O — a neutral complex, no ions outside the bracket; i = 1
[Co(H2O)6]Cl3 — \( \ce{[Co(H2O)6]^{3+}} \) + \( \ce{3Cl^{-}} \); i = 4
[Co(H2O)5Cl]Cl2·H2O — \( \ce{[Co(H2O)5Cl]^{2+}} \) + \( \ce{2Cl^{-}} \); i = 3
[Co(H2O)4Cl]Cl2·2H2O — the complex ion + \( \ce{2Cl^{-}} \); i = 3
\( \ce{[Co(H2O)6]Cl3} \), with i = 4, gives the maximum depression of freezing point.
Why the other options are incorrect
Option (1) — Incorrect. All three chlorides are bonded to cobalt, so the compound does not ionise (i = 1); it gives the smallest depression.
Option (3) — Incorrect. Only two chloride ions are outside the bracket, so i = 3.
Option (4) — Incorrect. Again only two chloride ions are outside the bracket, so i = 3; the water of crystallisation adds no particles.