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

    Frequently Asked Questions

    What does pKa tell you?

    pKa indicates acid strength on a logarithmic scale. Lower pKa means stronger acid. Each unit decrease in pKa represents a 10-fold increase in acid strength.

    Can pKa be negative?

    Yes. Strong acids like HCl have negative pKa values (around -7), meaning they dissociate almost completely.

    How do I convert pKa back to Ka?

    Ka = 10^(-pKa). For example, pKa = 4.76 gives Ka = 10^(-4.76) = 1.74 × 10⁻⁵.

    Ready to run the numbers?

    Open pKa Calculator