In the given reaction, you have a copper complex that changes color from pale sky-blue to dark blue when ammonia is added. The reaction is as follows:
\[ \text{Cu(H}_2\text{O)}_4^{2+} (aq) + 4 \text{NH}_3(aq) \rightleftharpoons \text{Cu(NH}_3)_4^{2+} (aq) + 4 \text{H}_2\text{O}(aq) \]
To determine which change to a solution at equilibrium will make the solution darker (i.e., favor the formation of the dark blue complex \(\text{Cu(NH}_3)_4^{2+}\)), we can apply Le Chatelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.
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Removing ammonia (NH3): This will shift the equilibrium to the left to produce more ammonia, thereby resulting in less \(\text{Cu(NH}_3)_4^{2+}\) and moving toward the pale sky-blue complex.
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Adding water (H2O): This will dilute the solutions and can also shift the equilibrium to the left, leading to a decrease in the dark blue complex.
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Removing \(\text{Cu(NH}_3)_4^{2+}\): This will also shift the equilibrium to the left to produce more of the dark blue complex, but overall it will result in less of that complex in the solution.
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Adding \(\text{Cu(NH}_3)_4^{2+}\): This will directly increase the concentration of the dark blue complex, shifting the equilibrium to the right and leading to a darker solution.
In summary, the change that will make the solution darker is:
Adding \(\text{Cu(NH}_3)_4^{2+}\) salt.