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A weak acid is an acid that does not ionize in solution to a significant extent; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains. Weak acids in water dissociate as
The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (K<sub>a</sub>):
The greater the value of K<sub>a</sub>, the more the formation of H<sup>+</sup> is favored, and the lower the pH of the solution. The K<sub>a</sub> of weak acids varies between 1.8×10<sup>-16</sup> and 55.5. Acids with a K<sub>a</sub> less than 1.8×10<sup>-16</sup> are weaker acids than water. Acids with a K<sub>a</sub> of greater than 55.5 are strong acids and almost totally dissociate when dissolved in water. The vast majority of acids are weak acids. Organic acids are a large subset of weak acids. Common household weak acids include acetic acid found in vinegar, and citric acid found in lemons.