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Question:

If the freezing point of a 0.01 molal aqueous solution of a cobalt(III) chloride-ammonia complex (which behaves as a strong electrolyte) is -0.0558 °C, the number of chloride(s) in the coordination sphere of the complex is: [Kf of water = 1.86 K kg mol⁻¹]

Solution:

The Vant Hoff factor is
i = ΔTf/(Kf × m) = 0.0558/(1.86 × 0.01) = 3
Thus, the complex dissociates to give 3 ions.
Hence, the complex is [Co(NH3)5Cl]Cl2 with 1 chloride ion in the coordination sphere.
[Co(NH3)5Cl]Cl2 → [Co(NH3)5Cl]²⁺ + 2Cl⁻