Kb is the base dissociation constant, a quantitative measure of how completely a weak base accepts a proton (H⁺) from water. For B + H₂O ⇌ BH⁺ + OH⁻, Kb = [BH⁺][OH⁻] / [B]. A larger Kb means a stronger base.

What units does Kb have?

Kb is technically dimensionless when thermodynamic activities are used, but in practice it carries units of mol/L (M) because concentrations are measured relative to the 1 M standard state.

How is pH calculated from Kb?

The tool first finds [OH⁻] by solving x² + Kb·x − Kb·C = 0 (quadratic formula: x = (−Kb + √(Kb² + 4·Kb·C)) / 2). Then pOH = −log₁₀([OH⁻]) and pH = 14 − pOH at 25 °C.

Why use the quadratic formula instead of the simple approximation?

The simple approximation [OH⁻] ≈ √(Kb·C) overestimates accuracy when ionization exceeds about 5%. The quadratic formula gives an exact result regardless of how large Kb is relative to C.

How are Ka and Kb related?

For a conjugate acid-base pair, Ka × Kb = Kw (≈ 1.0 × 10⁻¹⁴ at 25 °C). Equivalently, pKa + pKb = 14. Use the Ka Calculator on this site to work with the acid side of the equilibrium.

What is a typical Kb range for weak bases?

Weak bases typically have Kb values between 10⁻² and 10⁻¹², corresponding to pKb values of 2 to 12. Ammonia has Kb ≈ 1.8 × 10⁻⁵ (pKb 4.74).

Kb Calculator

Name: Kb Calculator
Availability: InStock
Author: Nham Vu

Calculate Kb from equilibrium concentrations of a weak base, or find the pOH and pH of a weak base solution given its Kb and initial concentration.

For the reaction B + H₂O ⇌ BH⁺ + OH⁻, enter equilibrium concentrations:

[OH⁻] — hydroxide concentration (mol/L)

[BH⁺] — conjugate acid concentration (mol/L)

[B] — undissociated base concentration (mol/L)

Enter values and click Calculate to see results.

Summary

Calculate Kb from equilibrium concentrations of a weak base, or find the pOH and pH of a weak base solution given its Kb and initial concentration.

How it works

Choose a calculation mode: "Find Kb" or "Find pH from Kb".
In Find Kb mode, enter the equilibrium concentrations of [OH⁻], [BH⁺], and [B].
In Find pH mode, enter the Kb value and the initial base concentration (C).
Click Calculate to see Kb (or pH), pOH, pKb, percent ionization, and the full equilibrium expression.
Use the Copy button to transfer the primary result to the clipboard.

Use cases

Determine Kb of ammonia, methylamine, or any weak base from lab measurements.
Predict the pH of an ammonia or amine solution given its Kb.
Verify equilibrium constant calculations for general chemistry homework.
Compare base strengths by computing Kb for different bases.
Support analytical chemistry titration analysis and buffer design.
Study acid-base equilibria in biochemistry and pharmacology.
Convert between Kb and pKb for reference or reporting.
Check percent ionization to confirm whether the weak-base approximation is valid.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu