Among the acids listed, which is the strongest in water?

• A. Acetic acid
• B. Carbonic acid
• C. Nitric acid
• D. Phosphoric acid

Answer: (C)

In water, how strong an acid is depends on how completely it donates a proton. A strong acid dissociates essentially fully, giving a high concentration of hydronium ions. Nitric acid is such a strong acid; in aqueous solution it largely ionizes to hydronium and nitrate: HNO3 + H2O → H3O+ + NO3−. The others are weak acids that only partially dissociate: acetic acid has a relatively small Ka (pKa about 4.76), carbonic acid has a very small Ka for the first dissociation (pKa around 6.35), and phosphoric acid’s first dissociation has a pKa around 2.15 but still does not proceed to completion at typical concentrations. Because nitric acid ionizes almost completely, it produces the highest [H3O+] for a given concentration, making it the strongest acid among those listed in water.