Posted by Maggy on .
You have amisture that is 0.100mol/L in each of the following ions: Cl, Br, and I.
(a) Which substance will precipitate first?
AgCl, AgBr, AgI?
(b)what is the concentration of Ag+ required to start the precipitation of the first substance?
ksp (AgCl)= 1.77*10^10
ksp (AgBr)= 5.35*10^13
ksp (AgI)= 8.51*10^17
Could you please help me understand how to do this problem? I am so lost.

Chemistry 
DrBob222,
Look up Ksp for AgCl, AgBr and AgI.
The salt with the smallest Ksp will ppt first.(this comparison is true only when the salts have the same ratio. In this case all are 1:1).
The following shows you how to calculate Ag^+ at the time EACH ppts. However, AgCl will not ppt first and that's the one you are calculate. AgI ppts first.
......AgCl(s) ==> Ag^+(aq) + Cl^(aq)
I.......solid......0..........0
C.......solid......x..........x
E.......solid......x..........x
Ksp = whatever = (Ag^+)(Col^)
Ksp = (x)(x)
Solve for x = (Ag^+). 
Chemistry 
Maggy,
i got 4.46*10^4??

Chemistry 
DrBob222,
No, that isn't right; however, I may have led you down the wrong path. I see at the top that the solution is 0.1M in I^ so
AgI==> Ag^+ + I^
........x.....x
Ksp = (Ag^+)(I^)
Ag^+ is x from AgI.
I^= 0.1M from the problem.
8.51E17 = (0.1)(x)
Solve for x.