Kb is the base dissociation constant — a measure of how completely a weak base accepts a proton from water. Larger Kb means a stronger weak base. For the reaction B + H₂O ⇌ BH⁺ + OH⁻, Kb = [BH⁺][OH⁻] / [B].

Why use the quadratic formula instead of the approximation?

The square-root approximation [OH⁻] ≈ √(Kb·C) is only reliable when dissociation is below ~5%. This calculator uses the exact quadratic so it works for any valid Kb and concentration combination.

Why is pH = 14 − pOH?

At 25°C, Kw = [H⁺][OH⁻] = 10⁻¹⁴, so pH + pOH = 14. This lets you convert between [OH⁻] and [H⁺] without extra steps.

What are typical Kb values for common weak bases?

Ammonia (NH₃) has Kb ≈ 1.8 × 10⁻⁵; methylamine ≈ 4.4 × 10⁻⁴; pyridine ≈ 1.7 × 10⁻⁹; aniline ≈ 4.3 × 10⁻¹⁰.

How does Kb relate to Ka?

For a conjugate acid-base pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25°C. If you know Ka for the conjugate acid, Kb = Kw / Ka.

Can I calculate Kb from a measured pH?

Yes — switch to "Solve for Kb" mode. Enter the initial concentration and the measured pH; the calculator derives [OH⁻] from pH, then computes Kb = [OH⁻]² / (C − [OH⁻]).

Base Dissociation Calculator

Name: Base Dissociation Calculator
Availability: InStock
Author: Nham Vu

Enter Kb and initial concentration to find pOH, pH, [OH⁻], and percent dissociation of a weak base.

Weak Base Parameters

Base Dissociation Constant (Kb)

× 10 (exponent)

Example: ammonia Kb = 1.8 × 10⁻⁵

Initial Concentration C (mol/L)

mol/L

Quick Presets

Results

Enter Kb and concentration, then click Calculate.

Equilibrium Chemistry

Reaction

B + H₂O ⇌ BH⁺ + OH⁻

Kb = [BH⁺][OH⁻] / [B]

Quadratic Form

x² + Kb·x − Kb·C = 0

x = [OH⁻] (positive root)

pH Conversion

pOH = −log₁₀([OH⁻])

pH = 14 − pOH (25°C)

Summary

Enter Kb and initial concentration to find pOH, pH, [OH⁻], and percent dissociation of a weak base.

How it works

Enter the base dissociation constant (Kb) as mantissa and exponent.
Enter the initial concentration (C) of the base in mol/L.
The calculator solves x² + Kb·x − Kb·C = 0, where x = [OH⁻].
pOH = −log₁₀([OH⁻]) and pH = 14 − pOH.
Degree of dissociation α = [OH⁻] / C × 100%.
Switch to "Solve for Kb" mode to find Kb from concentration and measured pH.

Use cases

Find the pH of an ammonia solution in general chemistry coursework.
Verify the weak-base approximation (x ≪ C) for a given Kb and concentration.
Compare base strength across different Kb values at the same concentration.
Back-calculate Kb from experimental pOH measurements.
Prepare buffer solutions with a known base component.
Analyze degree of dissociation when diluting a weak base.

Frequently Asked Questions

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