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Which of the following complex ions would you expect to have the lowest [Ag+] in a solution that is 0.10 M in the complex ion and 1.0 M in the free ligand? Explain.
Let L = ligand in each case, so the formation reaction is
Ag+(aq) + 2 Lm(aq)[AgL2](m1)(aq)
The mass action expression is:
Now the silver ion concentration can be calculated for each case:
(a) Kf = 1.6×107 so [Ag+] = [0.10]/(1.6×107)[1.0]2 = 6.3×109 M
(b) Kf = 5.6×1018 so [Ag+] = [0.10]/(5.6×1018)[1.0]2 = 1.8×1020 M
(c) Kf = 1.7×1013 so [Ag+] = [0.10]/(1.7×1013)[1.0]2 = 5.9×1015 M
The [Ag(CN)2] solution has the lowest silver ion concentration. Since all of these complex ions have the same stoichiometry, a comparison of the formation constants would have sufficed to answer the question: the largest formation constant leads to the lowest silver ion concentration.