An unknown sample is analyzed using the general qualitative analysis scheme shown in Figure 18.16. The sequence of reagents shown in the figure are used and a precipitate in only produced when (NH₄)₂HPO₄ in NH₃ is added. Which of the following describes a set of ions that might be present in the solution?

• A. Mg²⁺
• B. Ba²⁺ and Mg²⁺
• C. Ba²⁺, Mg²⁺ and Na⁺
• D. Ag⁺, Ba²⁺, Mg²⁺ and NH₄⁺

Answer: (B)

Verified step by step guidance

1. Identify the reagent that causes the precipitation: (NH4)2HPO4 in NH3.
2. Understand that (NH4)2HPO4 in NH3 is used to precipitate certain metal ions as their phosphates.
3. Recall that Ba2+ and Mg2+ can form insoluble phosphates, while Na+ and NH4+ do not form precipitates with phosphate.
4. Consider the solubility rules: Ba3(PO4)2 and Mg3(PO4)2 are insoluble in water, leading to precipitation.
5. Conclude that the presence of Ba2+ and Mg2+ ions in the solution is consistent with the observed precipitation when (NH4)2HPO4 in NH3 is added.