What is the Ksp expression for MgCO3 in water?

• A. [Mg2+][CO3^2-]
• B. [MgCO3]
• C. [Mg2+]^2[CO3^2-]
• D. [CO3^2-]^2

Answer: (A)

The key idea is that the solubility product (Ksp) comes from the dissolution equilibrium with the solid present. For magnesium carbonate, the solid MgCO3(s) dissolves as Mg2+(aq) and CO3^2−(aq) in a 1:1 ratio: MgCO3(s) ⇌ Mg2+(aq) + CO3^2−(aq). At equilibrium, the Ksp expression is the product of the two ion activities (approximated by concentrations in dilute solutions): Ksp = [Mg2+][CO3^2−]. The solid is omitted because its activity is taken as 1. Since the dissolution involves one magnesium ion for every carbonate ion, the two concentrations are multiplied directly, not squared or combined in any other way. The other forms would misrepresent the stoichiometry or include the solid, which is not part of the expression. Hydrolysis of carbonate can affect the actual ion concentrations, but the Ksp expression itself remains the product of the ion concentrations that are in equilibrium with the solid.