Base Dissociation Constant Calculator

Work out base dissociation constant instantly with clear inputs, formula shown and shareable results.

About Base Dissociation Constant Calculator

A weak base is handled through pOH rather than pH: pOH = pKw - pH gives the hydroxide concentration, which equals the protonated base formed. Kb is then [OH-]^2 over the base remaining. A 0.1 M base reading pH 11.13 has [OH-] = 1.35e-3 and Kb near 1.8e-5, matching ammonia.

Formulas

Equilibrium expression

Kb = [OH-][BH+] / [B] = [OH-]^2 / (C - [OH-])

Hydroxide from pH

[OH-] = 10^-(pKw - pH)

Worked example

With the default values below, Base Dissociation Constant Calculator returns Base Dissociation Constant Kb of 1.8446 × 10⁻⁵. Change any input above and every figure here is recalculated in your browser.

Inputs used

Initial base concentrationminimum 0 mol/L: 0.1 mol/L
Measured pHaccepts 0–16: 11.13
pKw at the measurement temperatureaccepts 10–16: 14

Results

Base Dissociation Constant Kb: 1.8446 × 10⁻⁵
pKb: 4.734
Degree Of Ionisation: 1.349%
Hydroxide Ion Concentration: 1.3490 × 10⁻³

Step by step

1Values used
2Equilibrium expression
3Hydroxide from pH
4Base Dissociation Constant Kb= 0.0000
5pKb= 4.734
6Degree Of Ionisation= 1.349
7Hydroxide Ion Concentration= 0.0013

Assumptions and limits

The result is derived only from the 3 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

Why not use pH directly?

Because a base's equilibrium is written in terms of hydroxide production. Converting through pKw keeps the expression honest.

What Kb would a strong base give?

The method breaks down for strong bases: they ionise fully, so C - [OH-] approaches zero and Kb is effectively infinite.