The basicity of an acid is the number of hydrogen ions that one molecule of the acid can release when it ionises in water.
Malay: Kebesan asid · Chinese: 酸的元数
The basicity of an acid is the number of hydrogen ions that one molecule of the acid can release when it ionises in water. It is a whole number fixed by the acid’s formula, and it tells you how many moles of H⁺ each mole of acid can supply. Basicity controls how much alkali is needed to neutralise the acid and how the salt is written.
A worked example lays out the range. Hydrochloric acid releases one H⁺ per molecule, HCl → H⁺ + Cl⁻, so it is monoprotic (basicity 1). Sulfuric acid releases two, H₂SO₄ → 2H⁺ + SO₄²⁻, so it is diprotic (basicity 2). Phosphoric acid, H₃PO₄, is triprotic with basicity 3. This is why one mole of sulfuric acid neutralises twice as much sodium hydroxide as one mole of hydrochloric acid, a point that decides the ratio in a titration calculation.
The confusion to avoid is reading “basicity” as anything to do with how basic or alkaline the acid is, it is not. Basicity counts the acidic hydrogens the molecule can give away, despite the word. A second slip is to count every hydrogen atom in the formula: ethanoic acid, CH₃COOH, has four hydrogen atoms but a basicity of only 1, because only the hydrogen in the –COOH group ionises off. Count the ionisable hydrogens, not all of them.
Within this chapter basicity links the acid definition to neutralisation and titration, since it sets the mole ratio between acid and alkali. Our teachers show you how to read basicity straight from a formula so your titration ratios come out right the first time.
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