acid dissociation constant
Imagine a crowd of acid molecules in water, each deciding whether to hold onto its proton or let it go. Some acids are generous and release almost every proton; others are stingy and keep most of theirs. The acid dissociation constant, Ka, is the single number that captures how willing a particular acid is to give up its proton.
It is the equilibrium constant for an acid HA splitting into H+ and its leftover A- in water: Ka = [H+][A-] / [HA]. A large Ka means the top of the fraction wins — lots of protons released — so the acid is strong. A small Ka means most molecules stay intact, so the acid is weak. Because Ka values span many orders of magnitude, chemists often quote their logarithm, pKa, instead.
Ka matters because it lets you predict, not just observe, how acidic a solution will be: from Ka and the concentration you can calculate the pH, design a buffer, or judge which of two acids will dominate. The caveat is that Ka is fixed only at a given temperature — change the temperature and the constant shifts.
Acetic acid (in vinegar) has Ka ≈ 1.8 × 10⁻⁵, very small, so in solution most of it stays as whole molecules and only a tiny fraction releases protons — which is why vinegar is a weak acid.
A bigger Ka means a stronger acid; the number sets the position of the equilibrium.
For bases there is a matching base dissociation constant, Kb. For a conjugate acid–base pair the two are linked by Ka × Kb = Kw, the autoionization constant of water, so knowing one gives the other.