Question

I've a solution 0.002M of Pb(NO3)2:

What is the minimum concentration of iodide ions (I-) necessary to observe the precipitation of PbI2?
What mass of KI should be added to observe the precipitation of PbI2 from the 25 mL volume lead nitrate solution?

Answer 1

................Pb(NO3)2 ==> Pb^2+ + 2NO3^-
I...............0.002..................0............0
C............-0.002.............0.002.......0.002*2
E.................0.................0.002.......0.004

..........PbI2 ==> Pb^2+ + 2I^-
Ksp = (Pb^2+)(I^-)^2
Look up Ksp and substitute.
You know (Pb^2+) from the Pb(NO3)2 equilibrium.
Solve for (I^-) in mols//L. Convert to mols/25 mL. Convert to grams KI/25 mL.
Post your work if you get stuck.