When to split molecules into their ions for Kc

[Jaden Haskins 2E]

When solving for Kc from an equation, sometimes we are supposed to break a molecule down into its ions. For example, in Pb(No3)2 (aq) + 2KCl (aq) --> PbCl2 (s) + 2 KNO3 (aq), the equilibrium constant is 1/[Pb2+][Cl-]^2. How do we know when to break into ions or to just leave the molecules as is?

[Jeffrey Hablewitz 2I]

If an ion is in an aqueous phase, the equilibrium expression should use the concentration of the ion's constituent cation and anion. The KNO₃ in your example doesn't appear in the equilibrium expression because both K⁺ and NO₃^- are spectator ions and arent involved in the reaction. The reaction could be simplified to 2Cl^-(aq) + Pb²⁺(aq) --> PbCl₂(s)