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

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

The greater the value of K_a, the more the formation of H⁺ is favored, and the lower the pH of the solution. The K_a of weak acids varies between 1.8×10^-16 and 55.5. Acids with a K_a less than 1.8×10^-16 are weaker acids than water. Acids with a K_a 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.

See also

• Strong acid
• Weak base