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# Weak acid

A weak acid is an acid that does not ionise in a 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

$\mathrm{ HA_{(aq)} \, \leftrightarrow \, H^+\,_{(aq)} +\, A^-\,_{(aq)} }.$

The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (Ka):

$\mathrm{ K_a\, =\, \frac {[H^+\,][A^-\,]}{[HA]} }$

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 organic acids include acetic acid found in vinegar, and citric acid found in lemons; weak mineral acids include boric acid used as an antiseptic and eyewash and phosphoric acid that appears in many soft drinks.