Monoprotic acid

When an acid, HA, dissolves in water, some molecules of the acid 'dissociate' to form hydronium ions and the conjugate base, (A-), of the acid.

HA --> H+ + A-

It is understood that H + stands for the hydronium ion and that each species in this equilibrium may solvate to a greater or lesser extent. The acid dissociation constant is defined as

Ka = [H+][A-]/[HA]

H2O is omitted from these expressions because in dilute solution the concentration of water may be assumed to be constant ([H2O] = 1). Values of Ka vary over many orders of magnitude, so it is common to take the negative logarithm to base ten of the value - this is just for ease of writing and calculations really

pKa = -log(ka)

It is easier to compare the strengths of different acids by comparing pKa values as they vary over a much smaller range.