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Chapter 27 Appendix C: Dissociation Constants and pKa Values for Acids at 25°C

Name Formula K a1 pKa1 K a2 pKa2 K a3 pKa3 K a4 pKa4
Acetic acid CH3CO2H 1.75 × 10−5 4.756
Arsenic acid H3AsO4 5.5 × 10−3 2.26 1.7 × 10−7 6.76 5.1 × 10−12 11.29
Benzoic acid C6H5CO2H 6.25 × 10−5 4.204
Boric acid H3BO3 5.4 × 10−10* 9.27* >1 × 10−14* >14*
Bromoacetic acid CH2BrCO2H 1.3 × 10−3 2.90
Carbonic acid H2CO3 4.5 × 10−7 6.35 4.7 × 10−11 10.33
Chloroacetic acid CH2ClCO2H 1.3 × 10−3 2.87
Chlorous acid HClO2 1.1 × 10−2 1.94
Chromic acid H2CrO4 1.8 × 10−1 0.74 3.2 × 10−7 6.49
Citric acid C6H8O7 7.4 × 10−4 3.13 1.7 × 10−5 4.76 4.0 × 10−7 6.40
Cyanic acid HCNO 3.5 × 10−4 3.46
Dichloroacetic acid CHCl2CO2H 4.5 × 10−2 1.35
Fluoroacetic acid CH2FCO2H 2.6 × 10−3 2.59
Formic acid CH2O2 1.8 × 10−4 3.75
Hydrazoic acid HN3 2.5 × 10−5 4.6
Hydrocyanic acid HCN 6.2 × 10−10 9.21
Hydrofluoric acid HF 6.3 × 10−4 3.20
Hydrogen selenide H2Se 1.3 × 10−4 3.89 1.0× 10−11 11.0
Hydrogen sulfide H2S 8.9 × 10−8 7.05 1 × 10−19 19
Hydrogen telluride H2Te 2.5 × 10−3‡ 2.6 1 × 10−11 11
Hypobromous acid HBrO 2.8 × 10−9 8.55
Hypochlorous acid HClO 4.0 × 10−8 7.40
Hypoiodous acid HIO 3.2 × 10−11 10.5
Iodic acid HIO3 1.7 × 10−1 0.78
Iodoacetic acid CH2ICO2H 6.6 × 10−4 3.18
Nitrous acid HNO2 5.6 × 10−4 3.25
Oxalic acid C2H2O4 5.6 × 10−2 1.25 1.5 × 10−4 3.81
Periodic acid HIO4 2.3 × 10−2 1.64
Phenol C6H5OH 1.0 × 10−10 9.99
Phosphoric acid H3PO4 6.9 × 10−3 2.16 6.2 × 10−8 7.21 4.8 × 10−13 12.32
Phosphorous acid H3PO3 5.0 × 10−2* 1.3* 2.0 × 10−7* 6.70*
Pyrophosphoric acid H4P2O7 1.2 × 10−1 0.91 7.9 × 10−3 2.10 2.0 × 10−7 6.70 4.8 × 10−10 9.32
Resorcinol C6H4(OH)2 4.8 × 10−10 9.32 7.9 × 10−12 11.1
Selenic acid H2SeO4 Strong Strong 2.0 × 10−2 1.7
Selenious acid H2SeO3 2.4 × 10−3 2.62 4.8 × 10−9 8.32
Sulfuric acid H2SO4 Strong Strong 1.0 × 10−2 1.99
Sulfurous acid H2SO3 1.4 × 10−2 1.85 6.3 × 10−8 7.2
meso-Tartaric acid C4H6O6 6.8 × 10−4 3.17 1.2 × 10−5 4.91
Telluric acid H2TeO4 2.1 × 10−8‡ 7.68 1.0 × 10−11‡ 11.0
Tellurous acid H2TeO3 5.4 × 10−7 6.27 3.7 × 10−9 8.43
Trichloroacetic acid CCl3CO2H 2.2 × 10−1 0.66
Trifluoroacetic acid CF3CO2H 3.0 × 10−1 0.52
* Measured at 20°C, not 25°C.
‡ Measured at 18°C, not 25°C.