pKb is the negative base-10 logarithm of the base dissociation constant: pKb = −log₁₀(Kb). For ammonia, Kb = 1.8 × 10⁻⁵, so pKb = 4.74. If you only know the conjugate acid, use pKb = pKw − pKa, where pKw is 14.00 at 25 °C; ammonium's pKa of 9.25 gives 4.75.
Acid–base tables are inconsistent: some list Kb for amines, others give only the pKa of the protonated form, and lab data arrive as a measured pH or hydroxide concentration. This calculator turns any of those three starting points into pKb, and reports Kb in scientific notation, the conjugate acid's pKa and Ka, and a rough strength label. The pKw field defaults to 14.00 but can be changed for work at other temperatures, since the pKa + pKb relation holds only with the pKw of the same temperature. The equilibrium mode assumes a single monoprotic weak base in water with no other hydroxide source, so it does not apply to buffers, polyamines with overlapping steps, or concentrated solutions where activities differ from concentrations.
Choose Kb value, pKa of conjugate acid, or Concentration + [OH⁻].
Type Kb in e-notation (1.8e-5), the pKa, or the initial molarity and equilibrium hydroxide concentration.
Leave 14 for 25 °C or enter the value for your temperature.
pKb appears with Kb, the conjugate pKa and Ka, and the working lines.
Result pKb: 4.74
Kb: 1.80 × 10^-5
pKa of conjugate acid: 9.26
Ka of conjugate acid: 5.56 × 10^-10
Classification: Weak base
The textbook Kb of aqueous ammonia gives pKb 4.74, and the ammonium ion's pKa follows as 14 − 4.74.
Result pKb: 4.75
Kb: 1.78 × 10^-5
pKa of conjugate acid: 9.25
Ka of conjugate acid: 5.62 × 10^-10
Classification: Weak base
When a table lists only the conjugate acid, subtract its pKa from pKw.
Result pKb: 4.74
Kb: 1.82 × 10^-5
pKa of conjugate acid: 9.26
Ka of conjugate acid: 5.49 × 10^-10
Classification: Weak base
Kb = (1.34 × 10⁻³)² ÷ (0.10 − 0.00134) = 1.82 × 10⁻⁵.
pKb = −log₁₀(Kb); pKb = pKw − pKa; Kb = [OH⁻]² ÷ (C − [OH⁻])
KbpKapKwC[OH⁻]Because Ka × Kb = Kw for a conjugate pair, the logs add to pKw. In the equilibrium mode each base molecule that reacts makes one BH⁺ and one OH⁻, so both equal [OH⁻] and the unreacted base is C − [OH⁻]; water's own hydroxide is neglected, which is fine when [OH⁻] is well above 10⁻⁶ M.
For a conjugate acid–base pair, Ka × Kb = Kw, so pKa + pKb = pKw. At 25 °C pKw is 14.00, which is why ammonia (pKb 4.75) pairs with ammonium (pKa 9.25).
Yes. A smaller pKb means a larger Kb, so more of the base takes a proton from water. Methylamine at pKb 3.36 is a stronger base than ammonia at 4.75, and pyridine at about 8.8 is much weaker.
Compute pOH = pKw − pH, then [OH⁻] = 10^−pOH. A 0.10 M ammonia solution at pH 11.13 has pOH 2.87 and [OH⁻] about 1.35 × 10⁻³ M.
Water's self-ionisation is endothermic, so Kw rises as it warms. pKw is about 14.94 at 0 °C, 14.00 at 25 °C and 13.53 at 40 °C, and pKa + pKb follows it.
Use e-notation: 1.8e-5 for 1.8 × 10⁻⁵ or 4.4e-4 for methylamine. The result shows Kb back in × 10^ form so you can check it.
Bases stronger than hydroxide are levelled in water: they react completely, so their Kb cannot be measured there and quoted values come from other solvents or estimates.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 12). pKb Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/chemistry/chemistry-reference/pkb-calculator/
Tools Directory Editorial Team. “pKb Calculator.” Tools Directory, 12 Oct. 2026, https://tools.milunir.com/chemistry/chemistry-reference/pkb-calculator/. Accessed 28 Oct. 2026.
Tools Directory Editorial Team. “pKb Calculator.” Tools Directory. Last modified October 12, 2026. Accessed October 28, 2026. https://tools.milunir.com/chemistry/chemistry-reference/pkb-calculator/.
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