Question 16.15: A 0.20-mole quantity of CuSO4 is added to a liter of 1.20 M ...
A 0.20-mole quantity of CuSO_{4} is added to a liter of 1.20 M NH_{3} solution. What is the concentration of Cu^{2+} ions at equilibrium?
Strategy The addition of CuSO_{4} to the NH_{3} solution results in complex ion formation
Cu^{2+}(aq)+4NH_{3}(aq) \xrightleftharpoons[]{} Cu(NH_{3} )^{2+}_{4}(aq)From Table 16.4 we see that the formation constant (K_{f}) for this reaction is very large; therefore, the reaction lies mostly to the right. At equilibrium, the concentration of Cu^{2+} will be very small. As a good approximation, we can assume that essentially all the dissolved Cu^{2+} ions end up as Cu(NH_{3})^{2+}_{4} ions. How many moles of NH_{3} will react with 0.20 mole of Cu^{2+} ? How many moles of Cu(NH_{3})^{2+}_{4} will be produced? A very small amount of Cu^{2+} will be present at equilibrium. Set up the K_{f} expression for the preceding equilibrium to solve for [Cu^{2+}] .
Table 16.4 Formation Constants of Selected Complex Ions in Water at 25°C
| Complex Ion | Equilibrium Expression | Formation Constant (K_{f}) |
| Ag(NH_{3})^{+}_{2} | Ag^{+}+2NH_{3} \xrightleftharpoons[]{} Ag(NH_{3})^{+}_{2} | 1.5\times 10^{7} |
| Ag(CN)^{-}_{2} | Ag^{+}+2CN^{-} \xrightleftharpoons[]{} Ag(CN)^{-}_{2} | 1.0\times 10^{21} |
| Cu(CN)^{2-}_{4} | Cu^{2+}+4CN^{-} \xrightleftharpoons[]{} Cu(CN)^{2-}_{4} | 1.0\times 10^{25} |
| Cu(NH_{3})^{2+}_{4} | Cu^{2+}+4NH_{3} \xrightleftharpoons[]{} Cu(NH_{3})^{2+}_{4} | 5.0\times 10^{13} |
| Cd(CN)^{2-}_{4} | Cd^{2+}+4CN^{-} \xrightleftharpoons[]{} Cd(CN)^{2-}_{4} | 7.1\times 10^{16} |
| CdI^{2-}_{4} | Cd^{2+}+4I^{-} \xrightleftharpoons[]{} CdI^{2-}_{4} | 2.0\times 10^{6} |
| HgCl^{2-}_{4} | Hg^{2+}+4Cl^{-} \xrightleftharpoons[]{} HgCl^{2-}_{4} | 1.7\times 10^{16} |
| HgI^{2-}_{4} | Hg^{2+}+4I^{-} \xrightleftharpoons[]{} HgI^{2-}_{4} | 2.0\times 10^{30} |
| Hg(CN)^{2-}_{4} | Hg^{2+}+4CN^{-} \xrightleftharpoons[]{} Hg(CN)^{2-}_{4} | 2.5\times 10^{41} |
| Co(NH_{3})^{3+}_{6} | Co^{3+}+6NH_{3} \xrightleftharpoons[]{} Co(NH_{3})^{3+}_{6} | 5.0\times 10^{31} |
| Zn(NH_{3})^{2+}_{4} | Zn^{2+}+4NH_{3} \xrightleftharpoons[]{} Zn(NH_{3})^{2+}_{4} | 2.9\times 10^{9} |
