Answer
Na₂CO₃
Procedure
Considering the equation: CaCO₃(s) ↔ Ca₂(aq) + CO₂
The common-ion effect is used to describe the effect on an equilibrium when one or more species in the reaction are shared with another reaction. This effect will be based on Le Chatelier's principle.
In this case, a common ion will be the carbonate ion, which is a polyatomic ion with the formula of CO₃²⁻. This will come from the Na₂CO₃.
Given that the effect is affecting the reagent, the equilibrium will shift towards the products.