What is the primary reason that NH_3(aq) is added to the dissolved penny solution in this experiment?

• A. To neutralize excess acid in the solution.
• B. To form a complex ion with Cu^{2+}, allowing its separation from other ions.
• C. To oxidize Zn^{2+} to Zn^{3+}.
• D. To precipitate Cu^{2+} as copper(II) hydroxide.

Answer: (B)

Verified step by step guidance

1. Understand the context: In the experiment, a penny is dissolved in acid, producing a solution containing Cu^{2+} ions along with other ions.
2. Recall the role of NH_3 (ammonia) in aqueous solutions: NH_3 acts as a ligand that can coordinate with metal ions to form complex ions.
3. Identify the behavior of Cu^{2+} ions in the presence of NH_3: Cu^{2+} ions react with NH_3 to form a soluble complex ion, such as [Cu(NH_3)_4]^{2+}, which helps separate copper ions from other species in solution.
4. Consider why NH_3 is not used to neutralize acid or oxidize Zn^{2+}: NH_3 is a weak base but its main function here is complexation, not neutralization or oxidation.
5. Recognize that NH_3 does not precipitate Cu^{2+} as copper(II) hydroxide in this context because the formation of the complex ion keeps copper in solution rather than forming a precipitate.