Equilibrium and using ICE tables

Question

Solid Ca₃(PO₄)₂ is placed into 10.0 L of water. When equilibrium is established, the concentration of Ca²⁺ is 2.3 × 10⁻⁴ M. What is Kc for this equilibrium? Note Kc is sometimes called K. Ca₃(PO₄)₂ (s) ⇌ 3 Ca²⁺ (aq) + 2 PO₄³⁻ (aq)

Answer 1

Ca₃(PO₄)₂(s) <==> 3Ca²⁺(aq) + 2PO₄^3-(aq) .. equation for dissolution of Ca₃(PO₄)²

K = [Ca²⁺]³[PO₄^3-]² (note: Ca₃(PO₄)₂(s) is not included in the expression b/c it is a solid)

Since [Ca²⁺] = 2.3x10^-4 M, the [PO₄^3-] is 2/3 x 2.3x10^-4 M = 1.53x10^-4 M

K = (2.3x10^-4)³(1.53x10^-4)²

K = 2.85x10^-19