MX2	⇌	Ag2+	+	2X-
s		s		2s

K_sp = [M²⁺] [X^–]² = (S)(2S)² = 4S³

$$ \Rightarrow $$ K_sp = 4(0.5 × 10^–4)³ = 5 × 10^–13

In polyprotic acids the loss of second proton occurs much less readily than the first. Usually the K_a values for successive loss of protons from these acids differ by at least a factor of 10^–3.

$$ \therefore $$ K_a1 > K_a2

CH₃COOH ⇌ CH₃COO^– + H⁺

K_c = $${{{\left[ {C{H_3}CO{O^ - }} \right]\left[ {{H^ + }} \right]} \over {\left[ {C{H_3}COOH} \right]}}}$$

$$ \Rightarrow $$ [CH₃COOH] = $${{{3.4 \times {{10}^{ - 4}} \times 3.4 \times {{10}^{ - 4}}} \over {1.7 \times {{10}^{ - 5}}}}}$$

= 6.8 × 10^–3

M₂S ⇌ 2M⁺ + S^2–

K_sp = [M⁺]²[S^2–] = (2s)²(s) = 4s ³

K_sp = 4(3.5 × 10^–6)³ = 1.7 × 10^–16