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Ka from pH of Weak Acid Calculator

Reverse: Ka ≈ [H⁺]² / C₀ from a measured pH.

Back-calculate the acid-dissociation constant Ka of a weak monoprotic acid from a measured pH and the initial concentration C₀, using Ka ≈ [H⁺]² / (C₀ − [H⁺]). Also reports pKa.

Dr. Helena Ortiz By Dr. Helena Ortiz
Published Last reviewed 1 min read

Inputs

mol/L

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

Ka = [H⁺]² / (C₀ − [H⁺]); [H⁺] = 10^(−pH); pKa = −log₁₀ Ka.

In depth

Back-calculate the acid-dissociation constant Ka of a weak monoprotic acid from a measured pH and the initial concentration C₀, using Ka ≈ [H⁺]² / (C₀ − [H⁺]). Also reports pKa.