pH Calculator (from [H⁺])

pH = -log[H⁺]. Compute pH from hydrogen-ion concentration in mol/L.

Inputs

Result

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How to use this calculator

  • Enter [H⁺] in mol/L (often a small number like 1e-7).
  • Read pH and the calculated pOH and [OH⁻].

About this calculator

pH is a logarithmic measure of hydrogen-ion concentration: pH = −log[H⁺]. Pure water at 25°C has [H⁺] = 10⁻⁷ mol/L → pH = 7 (neutral). Acidic: pH < 7. Basic: pH > 7. The relationship pH + pOH = 14 holds at 25°C in water.

Frequently asked

Why does pure water have pH 7?+
Water self-ionizes slightly: H₂O ⇌ H⁺ + OH⁻. At 25°C, [H⁺] = 10⁻⁷ M, so pH = 7. Higher temperature shifts pH of pure water down (still neutral, but lower number).
Can pH be negative or above 14?+
Yes — strong acids like 12 M HCl have pH ≈ -1. Strong bases like 12 M NaOH have pH ≈ 15. Outside the 0-14 range is unusual but valid.
How is pH measured in practice?+
pH meters use a glass electrode that develops a voltage proportional to [H⁺]. Calibrate with known buffers.
Strong vs weak acid pH?+
Strong acid (HCl): pH = -log[acid]. Weak acid (acetic): use Ka or the Henderson-Hasselbalch equation.
Common pH examples?+
Stomach acid: 1-2. Lemon: 2-3. Coffee: 5. Pure water: 7. Blood: 7.4. Soap: 9-10. Bleach: 12-13.

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