Chemistry - Ionic Equilibrium Question with Solution | TestHub

A 0.1 M solution of ${\mathrm{Cu}}^{+}$ions is stirred with an excess of potassium cyanide sufficient to convert all the ${\mathrm{Cu}}^{+}$ions to the corresponding cuprocyanide complex ${\left[\mathrm{Cu}(\mathrm{CN}{)}_4\right]}^{-3}$ and in addition to providing the solution with an excess of ${\mathrm{CN}}^{-}$equal to 0.2 M . Calculate the maximum pH of the solution when the final solution is treated with hydrogen sulphide to maintain $\left[{\mathrm{H}}_2\mathrm{S}\right]=0.1\mathrm{M}$ and such that the precipitation of cuprous sulphide is prevented. The instability constant for ${\left[\mathrm{Cu}(\mathrm{CN}{)}_4\right]}^{-3}$ is $6.4\times 10^{-15}{\mathrm{M}}^4,{\mathrm{K}}_{\mathrm{a}\text{, overall }}$ of ${\mathrm{H}}_2\mathrm{S}=1.6\times 10^{-21}\mathrm{M},{\mathrm{K}}_{\mathrm{sp}}$ of ${\mathrm{Cu}}_2\mathrm{S}=2.56\times 10^{-27}{\mathrm{M}}^3$.