Which substance dissolves in water to produce an acidic solution?

• A. FeCl3
• B. Na2O
• C. NaC2H3O2
• D. NH3

Answer: (A)

When a substance dissolves to give an acidic solution, it’s usually because it creates hydronium ions (H3O+). A salt containing a highly charged metal cation, like iron(III), does this through hydrolysis of the metal ion in water. In solution, Fe3+ becomes a hexa-aqua complex [Fe(H2O)6]3+, which can release a proton from one of the coordinated water molecules to form [Fe(H2O)5(OH)]2+ and H+. This release of H+ increases the hydronium concentration, giving an acidic solution. The chloride ions simply accompany the metal ion and don’t determine the acidity. In contrast, the oxide reacts with water to form NaOH and makes the solution basic; sodium acetate hydrolyzes to produce acetate, which is basic; ammonia reacts with water to form NH4+ and OH-, also basic. Therefore, dissolving iron(III) chloride produces an acidic solution.