Hydrosulfuric acid H 2 S( aq ) was used for years in chemistry laboratories to p

Question

Hydrosulfuric acid H2S(aq) was used for years in chemistry laboratories to precipitate metal                    sulfides from 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 H2SaqKa1 = 9.5 x 10-8,                    Ka2 = 1.3 x 10-14, 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.

Explanation / Answer

H2S <=> H+ + HS-

HS- <=> H+ + S2-

Let [H+] = C, is maintained as a constant rather than a equilibrium-variable.

Ka1 = C[HS-]/0.1 = 10*C*[HS-]

=> [HS-] = Ka1/(10C)

Ka2 = [H+][S2-]/[HS-] = [S2-]*C/(Ka1/10c) =10C^(2)[S2-]/Ka1

=> Ka1*Ka2 = 10C^(2)[S2-] = 12.35*10^(-22)

=> C^(2)*[S2-] = 1.235*10^(-22)

Ksp = [Mn2+][S2-] = [Mn2+](1.235*10^(-22))/(C^2)

=> C^(2) = [Mn2+](1.235*10^(-22))/(3.0*10^(-11)) = 4.12*10^(-12)*[Mn2+]

C = Sqrt(4.12*10^(-12) * [Mn+2])

The required Hydrogen ion concentration required as asked, depends upon the concentration of manganese ions in solution given for metal precipitation.

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