OK so lets consider another equilibrium.
Here is an insoluble salt.
For this substance it might quite be as obvious
as to why the pH would affect the
solubility of this salt.
But F- is the conjugate
base of a weak acid.
Our weak acid would be HF
and this is the conjugate base
of HF.
As a base it will be involved in
its own equilibrium, so here it is
F- in water, which is an aqueous solution
and we end up with
the F- having an OH
in its equilibrium.
Now this equilibrium here
obviously going to be affected by pH because
as we change the pH we are
changing the amount of OH-.
So lets walk our way through this
consideration with some questions.
So both of these equilibria are involved.
We have got the lead fluoride, an insoluble salt dissolving.
We have the F- of that salt
undergoing ionization with water. We have the F- of that salt
undergoing ionization with water.