ChemWhiz ReferenceEquilibrium
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.
01Use this when
Setting up an ICE table for a weak acid or base, and converting Ka ↔ pKa or using Ka·Kb = Kw for a conjugate pair.
02Conditions
25 °C; Kw = 1.0×10-14.
03Watch out
Use Ka for the acid and Kb for the base; for a conjugate pair Ka·Kb = Kw, do not reuse the same K.
Weak-Acid Ka and Weak-Base Kb at 25 °C
A smaller Ka or Kb means a weaker acid or base; each ionization step of a polyprotic acid has its own Ka.
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| CH3COOH | Acetic acid | Weak acid | Single acid ionization | 1.8 × 10-5 | 4.76 | Outside listed reaction | Outside listed reaction |
| C6H5COOH | Benzoic acid | Weak acid | Single acid ionization | 6.5 × 10-5 | 4.19 | Outside listed reaction | Outside listed reaction |
| HCO3- | Bicarbonate ion | Amphiprotic ion | Ka2 | 4.68 × 10-11 | 10.33 | 2.34 × 10-8 | 7.63 |
| H2CO3 | Carbonic acid | Weak acid | Ka1 | 4.27 × 10-7 | 6.37 | Outside listed reaction | Outside listed reaction |
| HCNO | Cyanic acid | Weak acid | Single acid ionization | 3.47 × 10-4 | 3.46 | Outside listed reaction | Outside listed reaction |
| H2PO4- | Dihydrogen phosphate ion | Amphiprotic ion | Ka2 | 6.17 × 10-8 | 7.21 | 1.32 × 10-12 | 11.88 |
| FCH2COOH | Fluoroacetic acid | Weak acid | Single acid ionization | 2.57 × 10-3 | 2.59 | Outside listed reaction | Outside listed reaction |
| HCOOH | Formic acid | Weak acid | Single acid ionization | 1.77 × 10-4 | 3.75 | Outside listed reaction | Outside listed reaction |
| HCN | Hydrocyanic acid | Weak acid | Single acid ionization | 4.9 × 10-10 | 9.31 | Outside listed reaction | Outside listed reaction |
| HF | Hydrofluoric acid | Weak acid | Single acid ionization | 7.2 × 10-4 | 3.14 | Outside listed reaction | Outside listed reaction |
| HPO42- | Hydrogen phosphate ion | Amphiprotic ion | Ka3 | 2.14 × 10-13 | 12.67 | 1.62 × 10-7 | 6.79 |
| HOCl | Hypochlorous acid | Weak acid | Single acid ionization | 3.5 × 10-8 | 7.46 | Outside listed reaction | Outside listed reaction |
| HNO2 | Nitrous acid | Weak acid | Single acid ionization | 7.24 × 10-4 | 3.14 | Outside listed reaction | Outside listed reaction |
| H3PO4 | Phosphoric acid | Weak acid | Ka1 | 7.59 × 10-3 | 2.12 | Outside listed reaction | Outside listed reaction |
| C6H8O7 | Citric acid | Polyprotic weak acid | Ka1 | 7.41 × 10-4 | 3.13 | Outside listed reaction | Outside listed reaction |
| NH4+ | Ammonium ion | Conjugate acid | Single acid ionization | 5.56 × 10-10 | 9.25 | Outside listed reaction | Outside listed reaction |
| CH3NH3+ | Methylammonium ion | Conjugate acid | Single acid ionization | 2.27 × 10-11 | 10.64 | Outside listed reaction | Outside listed reaction |
| NH3 | Ammonia | Weak base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 1.8 × 10-5 | 4.74 |
| (CH3)2NH | Dimethylamine | Weak base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 5.4 × 10-4 | 3.27 |
| CH3NH2 | Methylamine | Weak base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 4.4 × 10-4 | 3.36 |
| CH3COO- | Acetate ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 5.56 × 10-10 | 9.25 |
| C6H5COO- | Benzoate ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 1.54 × 10-10 | 9.81 |
| CNO- | Cyanate ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 2.88 × 10-11 | 10.54 |
| CN- | Cyanide ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 2.04 × 10-5 | 4.69 |
| FCH2COO- | Fluoroacetate ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 3.89 × 10-12 | 11.41 |
| HCOO- | Formate ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 5.65 × 10-11 | 10.25 |
| ClO- | Hypochlorite ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 2.86 × 10-7 | 6.54 |
| NO2- | Nitrite ion | Conjugate base | Base hydrolysis | Outside listed reaction | Outside listed reaction | 1.38 × 10-11 | 10.86 |
Important Notes
- All values at 25 °C (298.15 K). Ka describes proton donation to water; Kb describes proton acceptance from water. Outside listed reaction means that column does not describe the aqueous acid or base reaction selected for this row; it does not mean the species or a property is unknown.
- Amphiprotic ions have both acid and base reactions. The Kb values for bicarbonate, dihydrogen phosphate, and hydrogen phosphate are derived from Kw divided by Ka of their respective conjugate acids: carbonic acid, phosphoric acid, and dihydrogen phosphate. Do not divide Kw by the Ka of the same amphiprotic ion.
- Carbonic acid uses the apparent first-ionization constant for dissolved CO2 plus H2CO3, as used in course buffer calculations. Citric acid lists only its first ionization here.
- 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. A polyprotic acid loses each proton in turn with Ka1 > Ka2 > Ka3.
- The Ionization Step column marks the successive protons of a polyprotic acid (phosphoric acid gives Ka1, Ka2, Ka3; carbonic acid gives Ka1, Ka2). Filter by Conjugate family to select a specific pair or ladder (acetic acid with acetate, or the full phosphoric-acid ladder), or by Type to group all weak acids, weak bases, and conjugates.
- ChemWhiz uses this one consistent value set across the site.
Verification references: ChemWhiz-curated weak-acid Ka values at 25 °C, consistent with OpenStax Chemistry 2e, Appendix H (Ionization Constants of Weak Acids); ChemWhiz-curated weak-base Kb values at 25 °C, consistent with OpenStax Chemistry 2e, Appendix I (Ionization Constants of Weak Bases).