The strength of an acid or the measure of its tendency to release proton ions (H+) can be indicated from its dissociation constant which is called Ka.

The acid dissociation constant, pKa , is the negative logarithm of dissociation constant (Ka).

The dissociation constant of pure water is 1.9E-5.

An acid dissociation constant is the equilibrium constant for the disassociation of an acid, ultimately a measure of the acid's strength.

The dissociation constant is:
k = [H][X]/[HX]

The dissociation constant is:
k = [H][X]/[HX]