18. Hydrosulfuric solution. The H2S dissociates in two steps to form the S2-ion

Question

18. Hydrosulfuric solution. The H2S dissociates in two steps to form the S2-ion that then reacts with the metal ion in solution. By controlling the hydrogen ion concentration, the amount of sulfide in the solution can be controlled, and thus, very insoluble metal sulfides may be precipitated while other metals remain in solution. Given that for HaSa Kal -9.5 x 10, Kal -1.3 x 1014, Ksp for MnS 3.0 x 10-11, and that a saturated solution of H2S is approximately 0.100 M, calculate the hydrogen ion concentration required to prevent the precipitation of Mns. acid (H2Sag) was used for years in chemistry laboratories to precipitate metal sulfides from

Explanation / Answer

Ksp MnS = [Mn2+][S^2-] = 3 x 10^-11

H2S <==> H+ = HS-   

[H2S] = 0.1 M

let x amount of H2S has dissociated into x amount of H+ and x amount of HS-

Ka1 = [H+][HS-]/[H2S] = 9.5 x 10^-8

9.5 x 10^-8 = x^2/(0.1 - x)

with x being a small amount

[H+] = 3.08 x 10^-4 M

also,

HS- <==> H+ + S^2-

let x amount of HS- has dissociated in to H+ and S^2-

Ka2 = [H+][S^2-]/[HS-]

1.3 x 10^-14 = x^2/3.08 x 10^-4

x = [H+] = 1.14 x 10^-7 M

So the [H+] ion concentration to prevent precipitation of MnS

= 3.08 x 10^-4 M x 1.14 x 10^-7 M

= 3.08 x 10^-4 M

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