What is the effect of adding a strongly acidic salt like FeCl3 to water? What happens to pH?

• A. Hydrolysis of the metal cation yields H+, making the solution acidic; pH decreases.
• B. Hydrolysis of the chloride anion yields OH-.
• C. No hydrolysis occurs; solution remains neutral.
• D. The solution becomes basic.

Answer: (A)

The main idea is that highly charged metal cations acidify water through hydrolysis. In FeCl3, the Fe3+ ion is strongly polarizing to surrounding water molecules, so it tends to pull off a proton from the coordinated water: Fe(H2O)6^3+ ⇌ Fe(H2O)5(OH)^2+ + H+. This release of H+ increases the proton concentration, lowering the pH and making the solution acidic. The chloride ion, coming from a strong acid, acts as a nonreactive spectator in water and does not produce OH−, so it doesn’t counteract the acidity. Hence the solution becomes acidic and pH decreases.