# Why can’t I use solubility product for normally soluble compounds?

Solubility product only applies to sparing soluble ionic compounds (chemguide)

I read this in a website. But I don’t see what this should be the case. Taking $$\ce{NaCl}$$ for example, there’s an equilibrium established as it dissolves. Why then does solubility product not apply to this scenario?

• You could calculate it, but it's wouldn't be all that useful. – Mithoron Jul 1 at 14:49
• At high ion concentration, solubility products are activity driven and there are not good models for activity coefficients of such solutions. Using ion concentrations, the effective solubility product would be widely solution composition dependent. – Poutnik Jul 1 at 16:37

## 1 Answer

In addition to Mithoron's and Poutnik's comments, the solubility product may not be relevant to your solution chemistry anyway. Solubility products enter into solution equilibria when the precipitates are present, as in this answer where relative solubility products predict whether mostly magnesium or mostly iron will be dissolved in water when their hydroxides are present. With readily soluble materials, you likely do not get the precipitate so solubility products, even if you can compute then on an activity basis, have no bearing on the equilibrium.