What is the relationship between solubility s and solubility product K_sp for a sparingly soluble salt of type AB2 (like PbCl2)? (Hard)

Answer and explanation

Correct answer: B. K_sp = 4 s^3

  1. Step 1: Dissociation equilibrium: AB2(s) <=> A^(2+) + 2 B^(-). Step 2. [A^(2+)] = s, [B^(-)] = 2s. K_sp = [A^(2+)][B^(-)]^2 = (s)(2s)^2 = 4 s^3. Answer: K_sp = 4 s^3. Why not the others: s^2 is for 1:1 salts (AB); 27 s^4 is for 1:3 salts (AB3); 2 s^2 forgets squaring concentration.

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