Weak Base pH Calculator

How it works

B + H₂O ⇌ BH⁺ + OH⁻ · Kb = [BH⁺][OH⁻]/[B] · without water: x = [OH⁻] = (−Kb + √(Kb² + 4·Kb·C))/2 · shortcut x ≈ √(Kb·C) when ionization < 5% · pOH = −log[OH⁻] · pH = 14 − pOH (25 °C) · Ka·Kb = Kw = 1.0 × 10⁻¹⁴

A weak base accepts protons from water only partly, producing a small amount of hydroxide. The equilibrium constant Kb and the starting concentration fix how far the reaction goes. This calculator solves the full charge balance numerically, so it stays correct for very weak or very dilute bases where water’s own ionization matters, and it also shows the quick √(Kb·C) estimate taught in general chemistry along with the 5% rule for when it is acceptable. Kb values at 25 °C are from OpenStax Chemistry 2e, Appendix I.

Worked example

For 0.1 M ammonia (Kb = 1.8 × 10⁻⁵): solving Kb = x²/(0.1 − x) gives x = [OH⁻] = 1.33 × 10⁻³ M, so pOH = 2.88 and pH = 11.12. Only 1.333% of the ammonia is protonated. The √(Kb·C) shortcut gives [OH⁻] = 1.34 × 10⁻³ M and pH 11.13 — valid, because ionization is below 5%. pKb = 4.74 and the ammonium ion has Ka = 5.56 × 10⁻¹⁰.

Sources: OpenStax Chemistry 2e — Relative Strengths of Acids and Bases (Kb, ICE tables) · OpenStax Chemistry 2e — Appendix I: Ionization Constants of Weak Bases

Frequently asked questions