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pKb from Base Dissociation Constant

pKb=−log⁡10Kb\mathrm{p}K_b = -\log_{10} K_b

Units aren’t used in this calculation: every value is a plain number.

sig figs

Worked example: Methylamine Kb = 4.4e-4 → pKb = 3.35655. Press Try an example to run it live, then adjust anything.

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Where pKb from Base Dissociation Constant fits

Your answer feeds
Henderson–Hasselbalch Equation (Weak Base Buffer)
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See also

pH of a Weak Base from Kb: What the Kb is for: the pH of the base solution once you have it out of logarithmic form.Henderson–Hasselbalch Equation (Weak Base Buffer): Both use pkb.Ka and Kb Relation through Kw: Both use base dissociation constant.
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pKb from Base Dissociation Constant explained

KbpKb

Kb measures how far a base pulls a proton off water, and taking its negative logarithm produces the same compact scale that pKa gives on the acid side: small pKb means a strong base. Methylamine, with Kb = 4.4 × 10⁻⁴, has pKb = 3.36 and is a distinctly stronger base than ammonia at Kb = 1.8 × 10⁻⁵ and pKb 4.74: the extra electron density from the methyl group is worth more than a full order of magnitude.

Modern practice increasingly skips pKb entirely and quotes the pKa of the conjugate acid instead, since a single acid scale ranks everything from sulfuric acid to hydroxide without ever switching frameworks. The two are locked together by pKa + pKb = 14 at 25 °C, so a base of pKb 9.25 has a conjugate acid of pKa 4.75. Just remember that identity, like pH + pOH = 14, holds only near room temperature: Kw climbs with heat, and at 60 °C the sum is closer to 13.0.

pKb from Base Dissociation Constant formula

pKb=−log⁡10Kb\mathrm{p}K_b = -\log_{10} K_b
Where
  • pKb\mathrm{p}K_b= pKb
  • KbK_b= Base dissociation constant
In these toolkits

Acid–base relations

7 formulaspH, pOH, Kw, Ka and Kb and the weak-acid shortcut — the chain that turns a concentration into a pH and back again.

The buffer equations

4 formulasHenderson–Hasselbalch in both its acid and base forms, plus the pKa and pKb conversions that feed them — buffer pH from a ratio.

Revised 31 October 2026
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formula.expert. (2026, October 31). pKb from Base Dissociation Constant Formula & Calculator. https://formula.expert/solvers/pkb-from-kb

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