A solution contains 2.0 x 10 ^-4 M Ag+ and 1.5.....?

Question

A solution contains 2.0 x 10^ -4 M Ag+ and 1.5 x 10^-3 M Pb 2+. In NaI is added, will AgI (Ksp=8.3 x 10 ^-17) or PbI2 (Ksp= 7.9 x 10 ^-9) precipitate first? Specify the concentration of I need to begin precipitation

Answer 1

your first answer: AgI ppt's first

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AgI

K = [Ag] [I]

8.3e-17 = [2e-4][I]

[I] = 4.2e-13 molar

Your 2nd answer: to get AgI to ppt, [I] > 4.2e-13 Molar

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PbI2

K = [Pb] [I]^2

7.9e-9 = [1.5e-3] [I]^2

I2 = 5.26e-6

I = 2.3e-3 Molar

lastr answer:Pb needs the [I] > 2.3e-3 Molar