Which of the following pairs of cations cannot be separated by using an NH3 solution? (a) Pb2+, Zn2+ — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Ammonia solution acts as a precipitant and a complexing ligand. Step 2: When treated with excess ammonia, both Zn2+ and Cu2+ initially precipitate as hydroxides/basic salts but then readily dissolve to form stable soluble coordination complexes: [Zn(NH3)4]2+ (colorless) and [Cu(NH3)4]2+ (deep blue). Step 3: Because both remain in solution as soluble complexes, they cannot be separated from each other in one step using ammonia. Thus, option (c) is correct.