Weak acid
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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, (Ka):
![\mathrm{ K_a\, =\, \frac {[H^+\,][A^-\,]}{[HA]} }](../../../math/e/1/1/e1160cb29e6e647fa04612aee2af4f35.png)
The greater the value of Ka, the more the formation of H+ is favored, and the lower the pH of the solution. The Ka of weak acids varies between 1.8×10-16 and 55.5. Acids with a Ka less than 1.8×10-16 are weaker acids than water. Acids with a Ka 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.
