Question

What is the equilibrium constant for the dissolution of lead(II) chromate in Na2S2O3? For PbCrO4, Ksp = 2.0 x 10–16; for Pb(S2O3)34–, Kf = 2.2 x 106. (Use E for the power of 10)

Solid AgNO3 is slowly added to a solution that contains 0.24 M of Cl− and 0.10 M of Br− (assume volume does not change). What is [Br−] (in M) when AgCl(s) starts to precipitate? Ksp of AgCl is 1.8 x 10−10. Ksp for AgBr is 5.0 x 10−13. (Use E for the power of 10)

Answer 1

PbCrO4    ------------------> Pb+2 + CrO42-    Ksp = 2.0 x 10^–16

Pb+2    +   3 S2O3^2- ----------------> Pb(S2O3)4- , Kf = 2.2 x 10^6

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PbCrO4(s) + 3S2O3^2-(aq) <----------------> Pb(S2O3)3^4-(aq) + CrO4^2-(aq) .

.Keq = Ksp x Kf

Keq = 4.4 x 10^-10

b)

AgCl   -----------------> Ag+   + Cl -

Ksp = [Ag+][Cl-]

1.8 x 10^-10 = [Ag+] x 0.24

[Ag+] = 7.5 x 10^-10 M

Ksp = [Ag+][Br-]

5.0 x 10^-13 = 7.5 x 10^-10 x [Br-]

[Br-] = 6.7 x 10^-4 M