For this problem, we´re being asked to calculate the concentration of the lead ion Pb2+ that must be exceeded to precipitate PbF2 from a solution of fluoride ion F-.
In water, the dissociation of PbF2 is the following:
PbF2(s) ⇌ Pb2+(aq) + 2 F-(aq)
The Ksp expression for PbF2 is:
Where solids are ignored (reactants), so:
Note that each concentration is raised by the stoichiometric coefficient: [Pb2+] is raised to 1 and [F-] is raised to 2.
What concentration of the lead ion, Pb 2+, must be exceeded to precipitate PbF2 from a solution that is 1.00x10-2 M in the fluoride ion, F- = Ksp for lead(II) fluoride is 3.3x10-8.