The number of hydroxonium ions produced by one molecule of an acid in aqueous solution is its

Correct answer: basicity.

Why: The basicity of an acid is the number of hydronium ions (H₃O⁺) one molecule can produce in water (i.e., the number of replaceable/ionizable H⁺). For example, HCl is monobasic (1), H₂SO₄ is dibasic (2), H₃PO₄ is tribasic (3).

• Acid strength – Incorrect here. Strength is about the degree of ionization (how completely the acid dissociates), not how many H⁺ per molecule it can supply. E.g., HClO₄ is very strong but monobasic; H₃PO₄ is weak but tribasic.
• pH – Measures the hydrogen ion concentration of a particular solution; it depends on both concentration and strength.
• Concentration – Amount of acid per unit volume; unrelated to how many H⁺ a single molecule can release.