Posted by Karen on .
What is solubility of CaF2 in a solution buffered at pH 5
The acidity of the solution increases the solubility of CaF2 due to the formation of the weak acid, HF. The increase depends upon the pH, it may or may not be significant.
The two equations are:
CaF2 ==> Ca^+2 + 2F^-
HF ==> H^+ + F^-
Ksp = (Ca^+2)(F^-)^2
Ka = (H^+)(F^-)/(HF).
If (CaF2) = S, then (Ca^+2) = S and
2(Ca^+2) = (F^-).
2S = (F^-).
Solve Ka for HF.
(HF) = (H^+)(F^-)/Ka = (10^-5)(F^-)/Ka and rearrange to (HF) = some number*(F^-). Then
2S = (F^-) + (HF). solve for (F^-) and plug all this into Ksp and you will have only one unknown, namely S.
Post your work if you get stuck. Show what you are using for Ka and Ksp. Check my thinking.