A weak base only partially ionizes in water. Unlike strong bases (NaOH, KOH) that dissociate completely, weak bases reach an equilibrium between ionized and un-ionized forms, described by the dissociation constant Kb.

Kb is the base dissociation constant — a measure of how strongly a base ionizes in water. A larger Kb means stronger ionization (e.g., ammonia Kb ≈ 1.8 × 10⁻⁵). It is related to Ka by: Kb = Kw / Ka, where Kw = 1 × 10⁻¹⁴ at 25 °C.

Why does the calculator use the quadratic formula?

For accuracy with moderate Kb values or dilute solutions, the calculator solves x² + Kb·x − Kb·C = 0 exactly. The common approximation (x ≈ √(Kb·C)) is shown as a reference but may differ by >5% when Kb is not much smaller than C.

What temperature does this assume?

The calculation assumes 25 °C (298 K), where Kw = 1 × 10⁻¹⁴ and pH + pOH = 14. At other temperatures Kw changes, so the pH + pOH = 14 relationship no longer holds exactly.

What is percent ionization?

Percent ionization = ([OH⁻] / C) × 100. It shows what fraction of the base actually ionized. Weak bases typically have percent ionizations below 5%. Higher dilution increases percent ionization even though [OH⁻] falls.

pH of Weak Base Calculator

Name: pH of Weak Base Calculator
Availability: InStock
Author: Nham Vu

Enter a weak base concentration and Kb to instantly calculate pH, pOH, and percent ionization.

Weak Base Parameters

Concentration (C) mol/L

Molar concentration of the weak base solution

Base Dissociation Constant (K_b)

Use scientific notation: e.g. 1.8e-5 for 1.8 × 10⁻⁵

Common Weak Bases

Enter concentration and Kb, then click Calculate pH.

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Summary

Enter a weak base concentration and Kb to instantly calculate pH, pOH, and percent ionization.

How it works

Enter the molar concentration (C) of the weak base in mol/L.
Enter the base dissociation constant (Kb) for the base.
The calculator solves the equilibrium expression: x² / (C − x) = Kb for x = [OH⁻].
pOH is calculated as −log₁₀([OH⁻]), then pH = 14 − pOH.
Percent ionization is shown as (x / C) × 100.

Use cases

Calculate pH of ammonia (NH₃) solutions in lab preparation.
Verify weak base equilibrium problems for general chemistry courses.
Determine the degree of ionization for amines and other weak bases.
Compare strong vs. weak base pH at the same concentration.
Check buffer component concentrations before mixing.
Solve Kb-to-pH conversions on chemistry exams.

Frequently Asked Questions

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