An unknown white solid consists of two compounds, each containing a different cation. As suggested in the illustration, the unknown is partially soluble in water. The solution is treated with NaOH(aq) and yields a white precipitate. The part of the original solid that is insoluble in water dissolves in HCl(aq) with the evolution of a gas. The resulting solution is then treated with (NH₄)₂SO₄(aq) and yields a white precipitate.

(a) Is it possible that any of the cations Mg²⁺, Cu²⁺, Ba²⁺, Na+, or NH⁴⁺ were present in the original unknown? Explain your reasoning. 

(b) What compounds could be in the unknown mixture (that is, what anions might be present)?

Solution + KOH(aq). Solid + HCl(aq) white ppt solution + gas + (NH4)SO4(aq) white ppt