Unformatted text preview: species below its step and the metal ion is the predominate species above
its step. If we want all three metal ions to be present as cyanide complexes, then
the p(CN) must be less than 7.27, which is a concentration of CN– greater than
5.4!10–8M. For any p(CN) between 7.56 and 10.24, only the silver ion will
be present as a cyanide complex. This corresponds to CN– concentrations between
5.8!10–11 and 2.8!10–8 M.
5. Suppose you wish to prepare a precipitate of zinc sulfide, ZnS. To what range of pH levels
should you adjust the pH to minimize its solubility? Briefly defend your choice.
Any insoluble salt of a weak base, such as the sulfide ion, becomes more soluble at pH levels where
the weak base reacts to form its conjugate weak acid. A ladder diagram for sulfide (see figure on left)
shows that S2– is the predominate species only when the pH is greater than 13.9 (perhaps even
14.9); if we wish to minimize the solubility of ZnS, we need to maintain a pH more basic than
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