posted by Chandler .
If .100 M NaCl (aq) is added slowly to a beaker containing both .120 M AgNO3 (aq) and .150 M Pb(NO3)2 (aq) at 25C which will precipitate first, AgCl(s)PbCl2(s)?
AgCl ==> Ag^+ + Cl^-
Ksp = (Ag^+)(Cl^-)
(Cl^-) = Ksp/(Ag^+).
YOu know Ksp and Ag^+, solve for Cl^-
PbCl2 ==> Pb^2+ + 2Cl^-
Ksp = (Pb^2+)(Cl^-)^2
(Cl^-) = sqrt(Ksp/(Pb^2+)
You know Ksp and (Pb^2+), solve for Cl^-.
Now consider that you are dripping the NaCl into the beaker (slowly) containing BOTH salts, the first ppt will be the one that exceeds the Ksp first.