In a neutral solution at 25°C, the concentrations of H3O+ and OH- are

In a neutral solution at 25°C, the concentrations of H3O+ and OH- are

• A. H3O+ > OH-
• B. OH- > H3O+
• C. They are equal
• D. Cannot determine

Answer: (C)

The key idea is that neutral water at 25°C has equal hydronium and hydroxide concentrations because of water's autoionization and the constant Kw. Water self-ionizes to H3O+ and OH−, and their concentrations multiply to Kw, which is 1.0 × 10^-14 at 25°C. In a neutral solution, the amounts of acid and base balance, so [H3O+] equals [OH−]. Solving gives each concentration as 1.0 × 10^-7 M. Therefore, the two are equal. If one were larger, the solution would be acidic or basic, not neutral.

The key idea is that neutral water at 25°C has equal hydronium and hydroxide concentrations because of water's autoionization and the constant Kw. Water self-ionizes to H3O+ and OH−, and their concentrations multiply to Kw, which is 1.0 × 10^-14 at 25°C. In a neutral solution, the amounts of acid and base balance, so [H3O+] equals [OH−]. Solving gives each concentration as 1.0 × 10^-7 M. Therefore, the two are equal. If one were larger, the solution would be acidic or basic, not neutral.