pKa Calculator Formula

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

Use the pKa Calculator

Formulas Used

pKa

pka = -log10(ka)

Variables

VariableDescriptionDefault
kaKa (Acid Dissociation Constant)0.0000175

How It Works

How to Calculate pKa

Formula

pKa = −log₁₀(Ka)

Where:

  • pKa = negative base-10 logarithm of Ka
  • Ka = acid dissociation constant

    • A lower pKa indicates a stronger acid. pKa is more convenient than Ka for comparing acid strengths.

    Worked Example

    Acetic acid has Ka = 1.75 × 10⁻⁵.

    ka = 0.0000175
    1. 01pKa = -log₁₀(Ka)
    2. 02pKa = -log₁₀(1.75 × 10⁻⁵)
    3. 03pKa = 4.76

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