Acid Ionization Constants in Water at 25 °C

For a weak acid HA that ionizes in water according to the reaction: HA + H2O = H3O+ + A-, the base ionization constant Kb is defined as
       [H+] [A-]
 Kb = -------------
        [HA]
and pKa = - log Ka

Acid Formula pKa
Hydriodic HI ~-11
Hydrobromic HBr ~-9
Perchloric HClO4 ~-7
Hydrochloric HCl ~-7
Chloric HClO3 ~-3
Sulfuric (1) H2SO4 ~-2
Nitric HNO3 ~-1.3
Hydronium ion H3O+ 0.00
Urea acidium ion NH2CONH3+ 0.18
Iodic HIO3 0.80
Oxalic (1) H2C2O4 1.23
Sulfurous (1) H2SO3 1.81
Sulfuric (2) HSO4- 1.92
Chlorous HClO2 1.96
Phosphoric (1) H3PO4 2.12
Arsenic (1) H3AsO4 2.30
Chloroacetic CH2ClCOOH 2.85
Hydrofluoric HF 3.18
Nitrous HNO2 3.34
Formic HCOOH 3.75
Benzoic C6H5COOH 4.19
Oxalic (2) HC2O4- 4.19
Hydrazoic HN3 4.72
Acetic CH3COOH 4.75
Propionic CH3CH2COOH 4.87
Pyridinium ion HC5H5N+ 5.25
Carbonic (1) H2CO3 6.37
Sulfurous (2) HSO3- 6.91
Arsenic (2) H2AsO4- 7.03
Hydrosulfuric H2S 7.04
Phosphoric (2) H2PO4- 7.21
Hypochlorous HClO 7.53
Ammonium ion NH4+ 9.25
Hydrocyanic HCN 9.21
Carbonic (2) HCO3- 10.32
Methylammonium ion CH3NH3+ 10.64
Arsenic (3) HAsO42- 11.53
Hydrogen peroxide H2O2 11.62
Phosphoric (3) HPO42- 12.67
Water H2O 14.00