Dissociation of an acid can be written in several possible ways:

HA⁺ <=> A + H⁺
HA <=> A- + H⁺
HA^- <=> A^2- + H⁺

Note that in some cases the conjugate base (A, A^-, or A^2-) has a negative charge and in other cases it does not, but in all cases it has one less positive charge than the acid. For convenience, we will always write such reactions as HA <=>H⁺ + A^-. The equilibrium constant (Ka) for the dissociation of a weak acid (often called the dissociation constant) is defined as

Equation 2.8

The larger Ka is, the greater is the tendency for the acid to dissociate, that is, the stronger the acid.

The strength of acids is usually expressed in terms of the pKa value:

pKa = -log(Ka)