pH / acid-base calculator

Convert between pH, pOH, [H⁺] and [OH^-], or compute the pH of a strong or weak acid or base from its concentration. Uses K_w = 1.0 × 10^-14 at 25 °C.

Formulas

pH = -log₁₀([H⁺])

pOH = -log₁₀([OH^-])

[H⁺] × [OH^-] = K_w = 10^-14 → pH + pOH = 14

Strong acid / base. A strong acid fully dissociates, so [H⁺] equals its concentration — except in very dilute solutions, where water's own contribution matters. The solver uses [H⁺]² - c[H⁺] - K_w = 0 so the answer stays correct down to ~10^-8 M.

Weak acid. For HA ⇌ H⁺ + A^-, the equilibrium x² + K_ax - K_aC = 0 is solved exactly — no “x « C” approximation. Weak base mode uses the same shape with K_b and [OH^-] in place of K_a and [H⁺].

Common reference values at 25 °C: acetic acid K_a = 1.8 × 10^-5, ammonia K_b = 1.8 × 10^-5, formic acid K_a = 1.8 × 10^-4, hydrofluoric acid K_a = 6.6 × 10^-4.