Acid-Base Strengths and Conjugate Bases: pKa Values and Examples

NH3 (ammonia)

pKa ≈ 35 | Conjugate base: NH2− (amide)

ROH (alcohol)

pKa = 15-19 | Conjugate base: RO− (alkoxide)

H2O (water)

pKa = 14.0 | Conjugate base: HO− (hydroxide)

HPO4^2− (hydrogen phosphate)

pKa = 12.3 | Conjugate base: PO4^3− (phosphate)

RSH (thiol)

pKa = 10-12 | Conjugate base: RS− (thiolate)

R3NH+ (trialkylammonium ion)

pKa = 9-11 | Conjugate base: R3N (trialkylamine)

NH4+ (ammonium ion)

pKa = 9.25 | Conjugate base: NH3 (ammonia)

HCN (hydrocyanic acid)

pKa = 9.40 | Conjugate base: CN− (cyanide)

H2PO4− (dihydrogen phosphate)

pKa = 7.21 | Conjugate base: HPO4^2− (hydrogen phosphate)

H2S (hydrosulfuric acid)

pKa = 7.0 | Conjugate base: HS− (hydrosulfide)

RCOOH (carboxylic acid)

pKa = 4-5 | Conjugate base: RCOO− (carboxylate)

HF (hydrofluoric acid)

pKa = 3.2 | Conjugate base: F− (fluoride)

H3PO4 (phosphoric acid)

pKa = 2.2 | Conjugate base: H2PO4− (dihydrogen phosphate)

H3O+ (hydronium ion)

pKa = 0.0 | Conjugate base: H2O (water)

HNO3 (nitric acid)

pKa = −1.3 | Conjugate base: NO3− (nitrate)

p-Toluenesulfonic acid

pKa = −2.8 | Conjugate base: p-toluenesulfonate (tosylate)

H2SO4 (sulfuric acid)

pKa = −3 | Conjugate base: HSO4− (bisulfate)

HCl (hydrochloric acid)

pKa = −6 to −7 | Conjugate base: Cl− (chloride)

HBr (hydrobromic acid)

pKa = −8 to −9.5 | Conjugate base: Br− (bromide)

HI (hydroiodic acid)

pKa = −9.5 to −10 | Conjugate base: I− (iodide)

HClO4 (perchloric acid)

pKa = −10 | Conjugate base: ClO4− (perchlorate)