Common Ion Effect

If you intend to dissolve a salt in a solution the already contains one of the ions in this salt the solubility will be limited by the presence of this common ion (an ion common to both salts). For example, suppose we want to dissolve CaCO₃ in a 0.1M solution of Na₂CO₃. We know that K_sp for CaCO₃ is 4.9 x 10^-9. Let’s use s for the solubility of the salt. Then the concentration of Ca²⁺ will be s, but the concentration of CO₃^2- will be s + 0.1. So we have s(s + 0.1) = 4.9 x 10^-9. Since CaCO₃ is not very soluble we can substitute 0.1 for s + 0.1 (since s will be much smaller than 0.1) and get 0.1s = 4.9 x 10^-9. So the solubility of CaCO₃ will be 4.9 x 10^-8M. We can see that our substitution was valid as 4.9 x 10^-8 is much smaller than 0.1. We can also compare with the solubility of CaCO₃ in pure water (7.0 x 10^-5 M) and see that the presence of the common ion (CO₃^2-) has greatly reduced the solubility of the CaCO₃ (4.9 x 10^-8 < 7.0 x 10^-5).