Ka and Kb Values for Weak Acids and Bases
Acid-ionization (Ka) and base-ionization (Kb) constants at 25 °C for the weak acids, weak bases, and conjugate pairs used throughout the acid-base, buffer, hydrolysis, and titration topics. Use Ka × Kb = Kw = 1.0 × 10⁻¹⁴ to convert between a conjugate pair.
| Formula | Name | Type | Ka | pKa | Kb | pKb |
|---|---|---|---|---|---|---|
| CH3COOH | Acetic acid | Weak acid | 1.8 × 10-5 | 4.76 | — | — |
| C6H5COOH | Benzoic acid | Weak acid | 6.5 × 10-5 | 4.19 | — | — |
| HCO3- | Bicarbonate ion | Weak acid | 4.68 × 10-11 | 10.33 | — | — |
| H2CO3 | Carbonic acid | Weak acid | 4.27 × 10-7 | 6.37 | — | — |
| HCNO | Cyanic acid | Weak acid | 3.47 × 10-4 | 3.46 | — | — |
| H2PO4- | Dihydrogen phosphate ion | Weak acid | 6.17 × 10-8 | 7.21 | — | — |
| FCH2COOH | Fluoroacetic acid | Weak acid | 2.57 × 10-3 | 2.59 | — | — |
| HCOOH | Formic acid | Weak acid | 1.77 × 10-4 | 3.75 | — | — |
| HCN | Hydrocyanic acid | Weak acid | 4.9 × 10-10 | 9.31 | — | — |
| HF | Hydrofluoric acid | Weak acid | 7.2 × 10-4 | 3.14 | — | — |
| HPO42- | Hydrogen phosphate ion | Weak acid | 2.14 × 10-13 | 12.67 | — | — |
| HOCl | Hypochlorous acid | Weak acid | 3.5 × 10-8 | 7.46 | — | — |
| HNO2 | Nitrous acid | Weak acid | 7.24 × 10-4 | 3.14 | — | — |
| H3PO4 | Phosphoric acid | Weak acid | 7.59 × 10-3 | 2.12 | — | — |
| C6H8O7 | Citric acid | Polyprotic weak acid | 7.41 × 10-4 | 3.13 | — | — |
| NH4+ | Ammonium ion | Conjugate acid | 5.56 × 10-10 | 9.25 | — | — |
| CH3NH3+ | Methylammonium ion | Conjugate acid | 2.27 × 10-11 | 10.64 | — | — |
| NH3 | Ammonia | Weak base | — | — | 1.8 × 10-5 | 4.74 |
| (CH3)2NH | Dimethylamine | Weak base | — | — | 5.4 × 10-4 | 3.27 |
| CH3NH2 | Methylamine | Weak base | — | — | 4.4 × 10-4 | 3.36 |
| CH3COO- | Acetate ion | Conjugate base | — | — | 5.56 × 10-10 | 9.25 |
| C6H5COO- | Benzoate ion | Conjugate base | — | — | 1.54 × 10-10 | 9.81 |
| CNO- | Cyanate ion | Conjugate base | — | — | 2.88 × 10-11 | 10.54 |
| CN- | Cyanide ion | Conjugate base | — | — | 2.04 × 10-5 | 4.69 |
| FCH2COO- | Fluoroacetate ion | Conjugate base | — | — | 3.89 × 10-12 | 11.41 |
| HCOO- | Formate ion | Conjugate base | — | — | 5.65 × 10-11 | 10.25 |
| ClO- | Hypochlorite ion | Conjugate base | — | — | 2.86 × 10-7 | 6.54 |
| NO2- | Nitrite ion | Conjugate base | — | — | 1.38 × 10-11 | 10.86 |
Important Notes
- All values at 25 °C (298.15 K). A dash means the constant is not tabulated for that species — derive it from the conjugate partner with Ka × Kb = Kw.
- For a conjugate pair, pKa + pKb = pKw ≈ 14.00 at 25 °C (tabulated pairs may differ in the last digit from independent rounding).
- Larger Ka (smaller pKa) = stronger weak acid; larger Kb (smaller pKb) = stronger weak base.
- These are the course-canonical values: every ChemWhiz problem that needs one of these constants uses the value in this table.
Source: ChemWhiz course-canonical acid-base constants (25 °C), aligned with modern general-chemistry conventions