Question

If the Ksp of PbI₂ is 7.1 x 10^-9   will precipitation occur when 10 mL of 1.0 x 10^-4 Pb(NO₃)₂ is mixed with 10 mL of 1.0 x 10^-3 M KI? (Show calculation)

Answer 1

Pb(NO₃)₂ molarity in the mixture = molarity of Pb(NO₃)₂/ total volume

                                                      = 1.0 x 10^-4 / (10 +10)

                                                      = 5 x 10^-6 M

[ Pb+2 ] molarity = 5 x 10^-6 M

KI molarity in the mixture = 1.0 x 10^-3 / (10 + 10)

                                          = 5 x 10^-5 M

[I-] molarity = 5 x 10^-5 M

Q = ionic product = [Pb+2][I-]^2

                            = 5 x 10^-6 x (5 x 10^-5)^2

                             = 1.25 x 10^-14

Ksp of PbI₂ = 7.1 x 10-9

here Ksp > Q (ionic product). so precipitate will not form.