pH Calculator

pH from [H⁺], strong or weak acid and base concentrations (Ka or Kb), with pOH and [OH⁻].

pH = −log₁₀[H⁺]. For a weak acid, [H⁺] = (−Ka + √(Ka² + 4·Ka·C)) ÷ 2. 0.1 M acetic acid (Ka 1.8 × 10⁻⁵) has [H⁺] = 1.33 × 10⁻³ M, so pH = 2.88.

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How to use it

Choose what you know: a weak acid or base with Ka/Kb, a strong acid or base, [H⁺] or pH.
Enter the concentration in mol/L and the constant if needed (presets cover common acids).
Read pH, pOH, [H⁺] and [OH⁻].

pH = −log[H⁺] | pOH = 14 − pH | Weak acid: [H⁺] = (−Ka + √(Ka² + 4KaC))/2

Input: Weak base, C = 0.1 M, Kb = 1.8 × 10⁻⁵

Result: pH 11.12, [OH⁻] = 1.33 × 10⁻³ M

Only about 1.3% of the ammonia is protonated.

For a strong monoprotic acid, [H⁺] equals the acid concentration, so 0.01 M HCl has pH 2. For a weak acid, solve Ka = x² ÷ (C − x) for x = [H⁺].

At 25 °C, pH + pOH = 14 because [H⁺][OH⁻] = Kw = 1.0 × 10⁻¹⁴.

x ≈ √(Ka·C) works when the acid is less than 5% ionised. This calculator uses the exact quadratic so it is right either way.

Yes, for very concentrated strong acids above 1 M, although activity effects make simple calculations inaccurate there.