Plz help..im really stucked on this Ksp prob...i need to know ASAP?

Question

The solubility product constant of AgCl is 1.8 x 10^-10 and that of AgBr is 3.3 x 10^-13. Solid AgNO3 is slowly added to a solution containing 0.0010 M each of NaCl and NaBr. What concentration of Br- would remain in solution when AgCl beings to precipitate?

The solution only gave an answ of 1.8 x 10^-6 but there was no explanation...

Answer 1

AgCl will begin to precipitate when Q>K:

In other words,

AgCl----> Ag+ + Cl-

1.8 x10^-10<[Ag+][Cl-]                        

The solution already has chloride in it so the concentration of silver right as precipation begins is just 1.8 x 10^-10=X[.0010]

where X=1.8 x 10^7=[Ag+]          

When the concentration of silver is 1.8 x 10^-7, you can find Br- by just pluging that number into the ksp for AgBr

AgBr----> Ag+ + Br-

3.3 x 10^-13=[1.8x10^-7]X   X=1.8x10^-6=[Br-]