[Calgary Pure]

Hi,

I'm reading my textbook and the example they give for the Common Ion Effect on Solubility is to calculate the solubility of Ca(OH)₂ in 0.10 mol/L Ca(NO₃)₂. It is also given that the K_sp for Ca(OH)₂ = 6.5 x 10^-6.

They continue by assuming that the initial concentration of Ca²⁺ is 0.10 mol/L, the same as that of the Ca(NO₃)₂. This confuses me because wouldn't Ca(NO₃)₂ have an equilibrium point of its own, making the initial concentration of Ca²⁺ less than 0.1 mol/L when the solution contains only Ca(NO₃)₂.

Any help would be appreciated.

Thank you.

[Borek]

In general you are right, every substance do have such dissociation/dissolution equilibria. It is just that many of them are negligible.

Ca(NO₃)₂ is a strong electrolyte, fully dissociated. It is also quite well soluble, so at 0.1 M concentrations solution it is far from being saturated.