Acid Dissociation Constant
The equilibrium constant for the ionisation of an acid in water, measuring its strength.
Formula / equation
Ka = [H⁺][A⁻] / [HA]
Unit
mol/L
In depth
A larger Ka means a stronger acid and a more extensively ionised solution. Because values span more than twenty orders of magnitude, they are usually quoted logarithmically as pKa. Ka is temperature-dependent but independent of concentration.
Examples in the real world
Ethanoic acid has Ka = 1.8 × 10⁻⁵; hydrofluoric acid 6.8 × 10⁻⁴; hydrocyanic acid only 6.2 × 10⁻¹⁰.