Halide salt identified by sulfate solubility and silver halide behavior

Steps:

• Q dissolves in water and shows no reaction (no precipitate) with dilute H₂SO₄, ruling out Ba²⁺ (BaSO₄ insoluble) and confirming Mg²⁺ (MgSO₄ soluble).
• Q(aq) + AgNO₃ forms a precipitate, indicating halide ions (Br⁻ or I⁻) present, as AgBr and AgI are insoluble.
• The AgX precipitate is partially soluble in aqueous NH₃, matching AgBr (AgCl fully soluble, AgI insoluble).
• Q must contain Mg²⁺ and Br⁻, identifying it as MgBr₂.

Why C is correct:

• MgBr₂ is water-soluble, yields soluble MgSO₄ (no H₂SO₄ reaction), and forms AgBr (partially NH₃-soluble).

Why the others are wrong:

• A. Barium bromide: Forms insoluble BaSO₄ precipitate with H₂SO₄.
• B. Barium iodide: Forms insoluble BaSO₄ with H₂SO₄; AgI insoluble in NH₃.
• D. Magnesium iodide: AgI precipitate insoluble in NH₃.

Final answer: C