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A large number of molecules act as weak acids, forming some amount of
H3O+ ions when added to water. Unlike strong
acids, these molecules eventually form an equilibrium of the form
- HA + H2O < = > A- +
H3O+
The A- is known as the
conjugate base of the weak acid HA.
There are two common forms of weak acids
- Molecules with a ionizable hydrogen, such as HF, HCN or
CH3COOH.
- Cations such as NH4+ or many metal cations. The
metal cations form water complexes that can act as acids:
Al(H2O)6+3 + H2O < = >
Al(H2O)5OH+2 + H3O+
The strength of a weak acid is determined by its
ionization constant,
Ka |