pH dependent solubility

Question

pH dependent solubility

If you have a solution containing CaF2 the following reaction occurs:

CaF2 ➡️ Ca^2+ + 2F-

Why does adding HF cause the solubility of CaF2 to increase?

HF ➡️ H+ + F-

I know in this problem, the protons from the acid can combine with the F- ions from CaF2 to form HF. But I was thinking that since HF is bringing its own F ions into solution as well would that cause the common ion effect? Because of this I was thinking the pH change would be a net effect of zero since the acid is protonating some Fluoride ions but also bringing in its own into the solution. Hope that makes sense!

Answer 1

I'd have to agree with you on your interpretation. Certainly, adding an acid (H⁺) that didn't also contain the common ion (F^-) would increase the solubility, but I'm under the impression that adding both will not have that same effect.