How many moles of argon are in 12.6 g of argon?

According to the equilibrium equation for this reaction:

PbCl2(s)  ↔ Pb2+(aq)   +  2Cl-(aq)

So when Ksp is the solubility product constant for a solid substance when it

dissolved in the solution. and measure how a solute dissolves in the solution 

So, Ksp expression = [Pb2+] [Cl-]^2

and when the solubility is the maximum quantity of solute which can dissolve in a certain solute.
 
So, we assume the solubility = X 

∴[Pb2+] = X = 1.6 x 10^-2 M

[Cl-] = 2X = 2 * 1.6 x 10^-2 = 0.032 M

by substitution:

∴ Ksp = (1.6 x 10^-2) * (0.032)^2

           = 1.64 x 10^-5