pKa Calculator
Determine pKa or Ka via pKa = −log₁₀(Ka). Advanced mode finds pKa from pH and [HA] for a monoprotic weak acid, with step-by-step logic and an optional mini chart of acid strength.
Background
The acid dissociation constant Ka measures acid strength in water. Its logarithmic form pKa is defined by pKa = −log₁₀(Ka) and inversely Ka = 10⁻ᵖᴷᵃ. Lower pKa (or higher Ka) indicates a stronger acid.
How to use this calculator
- Quick convert: Enter Ka or pKa; the other is computed.
- Advanced: Enable Advanced and enter pH and initial [HA] to compute Ka and pKa for a monoprotic weak acid.
- Formulas: pKa = −log₁₀(Ka); Ka = 10⁻ᵖᴷᵃ; [H⁺] = 10⁻ᵖᴴ; Ka = [H⁺]² / ([HA]₀ − [H⁺]).
- Interpretation: Lower pKa (higher Ka) → stronger acid.
Formula & Equation Used
Primary relation: pKa = −log₁₀(Ka)
Inverse: Ka = 10⁻ᵖᴷᵃ
Advanced (weak acid, monoprotic): [H⁺] = 10⁻ᵖᴴ, Ka = [H⁺]² / ([HA]₀ − [H⁺])
Example Problems & Step-by-Step Solutions
Example 1 — Find pKa for Ka = 1.8×10⁻⁵
pKa = −log₁₀(1.8×10⁻⁵) = 4.74
Example 2 — Find Ka for pKa = 9.25
Ka = 10⁻⁹·²⁵ = 5.62×10⁻¹⁰
Example 3 — Advanced: pH = 2.87 and [HA]₀ = 0.100 M
[H⁺] = 10⁻²·⁸⁷ = 1.35×10⁻³ M; Ka = (1.35×10⁻³)² / (0.100 − 1.35×10⁻³) ≈ 1.83×10⁻⁵; pKa ≈ 4.74.
Frequently Asked Questions
Q: What does a low pKa mean?
A low pKa indicates a strong acid — it donates protons readily and has a large Ka.
Q: Can I get pKa from pH?
Yes, for a monoprotic weak acid if you also know initial [HA]: use [H⁺] = 10⁻ᵖᴴ and Ka = [H⁺]² / ([HA]₀ − [H⁺]).
Q: Does temperature matter?
Yes. Ka depends on temperature; this calculator assumes standard conditions unless you provide experimental pH and [HA].
Q: What if [H⁺] ≥ [HA]₀?
That violates the weak-acid assumption (or indicates other acid sources). The Advanced mode requires [HA]₀ > [H⁺].
