Acid dissociation constant
Where;
Ka is the Acid dissociation constant,
[H+] is the concentration of hydronium ion,
[A-] is the concentration of the conjugate base,
[HA] is the concentraion of the 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.
Values of Ka vary over many orders of magnitude, so it is common to take the negative logarithm to base ten of the value.
It is easier to compare the strengths of different acids by comparing pKa values as they vary over a much smaller range.