This is the concentration of Ag+
in the mixed solution before
precipitation. You need the concentration left in solution after
precipitation has occurred. You could set up a quadratic equation, but because the solubility of AgCl is very low you can simplify the problem by assuming that virtually all the Cl-
is precipitated as AgCl.
How many moles of Cl-
So how many moles of Ag+
are precipitated, and how many moles are left in solution?
, what is the concentration of Cl-
? Verify that this is a very small fraction of the original Cl-
, so that the simplification is justified.