pKb Calculator Formula

Understand the math behind the pkb calculator. Each variable explained with a worked example.

Use the pKb Calculator

Formulas Used

pKb

pkb = -log10(kb)

Variables

VariableDescriptionDefault
kbKb (Base Dissociation Constant)0.000018

How It Works

How to Calculate pKb

Formula

pKb = −log₁₀(Kb)

Where:

  • pKb = negative base-10 logarithm of Kb
  • Kb = base dissociation constant

    • A lower pKb indicates a stronger base. pKa + pKb = 14 at 25°C for a conjugate pair.

    Worked Example

    Ammonia has Kb = 1.8 × 10⁻⁵.

    kb = 0.000018
    1. 01pKb = -log₁₀(Kb)
    2. 02pKb = -log₁₀(1.8 × 10⁻⁵)
    3. 03pKb = 4.74

    Frequently Asked Questions

    What does pKb tell you?

    pKb measures base strength on a logarithmic scale. Lower pKb means stronger base. Each unit decrease represents a 10-fold increase in base strength.

    How are pKa and pKb related?

    For a conjugate acid-base pair at 25°C, pKa + pKb = 14. This comes from Ka × Kb = Kw = 10⁻¹⁴.

    How do I convert pKb back to Kb?

    Kb = 10^(-pKb). For example, pKb = 4.74 gives Kb = 10^(-4.74) = 1.82 × 10⁻⁵.

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