How to calculate pH

For a strong acid, it fully dissociates, so [H⁺] equals the concentration and pH = −log[H⁺]. For a strong base, [OH⁻] equals the concentration, pOH = −log[OH⁻], and pH = 14 − pOH. Weak acids only partly dissociate: set up Ka = x²/(C − x) where x = [H⁺] and Ka = 10^(−pKa), then solve — the shortcut x ≈ √(Ka·C) works when dissociation is small, and the exact quadratic works always. Weak bases work the same way with Kb and [OH⁻]. This tool solves the exact quadratic so it stays accurate even for concentrated or fairly strong weak acids where the approximation breaks down.

Assumes 25 °C (K_w = 1×10⁻¹⁴) and a monoprotic acid or base. pKa and pKb relate by pKa + pKb = 14 for a conjugate pair.

Related tools: Buffer calculator · all biochem tools.