pKa And pKb Calculator

Work out pka and pkb instantly with clear inputs, formula shown and shareable results.

About pKa And pKb Calculator

pKa is the negative logarithm of Ka, and an acid and its conjugate base are locked together by Ka x Kb = Kw, which in logarithmic form is pKa + pKb = pKw. Acetic acid with Ka = 1.8e-5 has pKa 4.74, so its conjugate base has pKb 9.26 at 25 C.

Formulas

Logarithmic constants

pKa = -log10(Ka), pKb = -log10(Kb)

Conjugate pair relation

pKa + pKb = pKw (14.00 at 25 C)

Worked example

With the default values below, pKa And pKb Calculator returns pKa of 4.745. Change any input above and every figure here is recalculated in your browser.

Inputs used

Acid dissociation constant Kaminimum 0: 0.000018
Ion product of water Kwminimum 0: 0

Results

pKa: 4.745
pKb Of The Conjugate Base: 9.255
Kb Of The Conjugate Base: 5.5556 × 10⁻¹⁰
pKw At This Temperature: 14.000

Step by step

1Values used
2Logarithmic constants
3Conjugate pair relation
4pKa= 4.745
5pKb Of The Conjugate Base= 9.255
6Kb Of The Conjugate Base= 0.0000
7pKw At This Temperature= 14.000

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Does a small pKa mean a strong acid?

Yes. A lower pKa means a larger Ka and more complete dissociation, so strong acids sit at negative pKa values.

Why is pKw shown separately?

Because the 14.00 figure only holds at 25 C. Changing Kw changes the pKa-pKb sum, and the calculator makes that explicit.