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A weak acid is an acid that dissociates incompletely and does not release all of its hydrogens in a solution i.e it does not completely donate all of its protons. These acids have higher pKa than strong acids, which release all of their hydrogens when dissolved in water.
While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid (H(CHB11Cl11), which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve, among other things, glass and all metals except iridium.
Weak acids ionize in water solution to only a moderate 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 acid dissociation constant expression, (Ka):
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.