A weak acid is defined by its lower level of ionization with the surrounding solution than strong acids, meaning that it has a lower tendency to exchange protons. Examples of weak acids include ethanoic acid, carbonic acid, hydrogen fluoride and acetic acid.
The ability of an acid to ionize is affected by the size and polarity of the atoms. The weakness of an acid is measured by the equilibrium acid dissociation constant K or by the pH measurement, depending on the acid's concentration. Calculations used to determine the strength or weakness of an acid include measures of electronegativity, atomic radius, equilibrium and charge.