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

Analyze the question: It asks for the term describing the number of hydroxonium ions (H₃O⁺) that one molecule of an acid produces when dissolved in water.

Concept: Acids are classified by how many H₃O⁺ ions each molecule can release in water. This number is called the basicity of the acid.

• HCl + H₂O → H₃O⁺ + Cl⁻ gives 1 H₃O⁺, so it is monobasic.
• H₂SO₄ + 2H₂O → 2H₃O⁺ + SO₄²⁻ gives 2 H₃O⁺, so it is dibasic.
• H₃PO₄ gives 3 H₃O⁺, so it is tribasic.

Checking each option:

• A. Basicity: This is the number of replaceable hydrogen ions (H₃O⁺ in water) per molecule of acid. It matches the question.
• B. Acid strength: This is how completely an acid ionises in water (strong or weak), not the number of H₃O⁺ ions per molecule. For example, ethanoic acid is weak but still monobasic.
• C. pH: This is a measure of the H₃O⁺ concentration, -log[H₃O⁺], not a count per molecule.
• D. Concentration: This is the amount of acid per unit volume, such as mol/dm³.

Answer: A, basicity.

Exam tip: "Basicity" refers to the number of H⁺/H₃O⁺ ions per molecule, which tells you how many "bases" the acid can react with. Don't confuse it with strength, which depends on the degree of ionisation. Ethanoic acid (CH₃COOH) has four hydrogens but a basicity of 1, because only the carboxyl hydrogen is ionisable.