How it’s calculated
pH is the negative base-10 logarithm of the hydronium ion concentration in mol/L; pOH is the same for hydroxide. In water they always add up to pKw, which is 14.00 at 25 °C.
Example (OpenStax Chemistry 2e §14.2, “Calculation of pOH”): 0.0125 M KOH gives [OH⁻] = 0.0125 M, so pOH = −log(0.0125) = 1.903 and pH = 14.00 − 1.903 = 12.10 (basic).
| pH | Meaning (25 °C) |
|---|---|
| < 7 | Acidic |
| 7 | Neutral |
| > 7 | Basic |
Frequently asked questions
Can pH be negative?
Yes. A strong acid more concentrated than 1 mol/L has a negative pH; for example [H₃O⁺] = 12 M gives pH ≈ −1.08.
How do I find the pH of a weak acid?
Weak acids only partly ionize, so [H₃O⁺] is less than the acid molarity. Work out [H₃O⁺] from the acid’s Ka first, then enter it here.
Why does pKw change with temperature?
Water’s self-ionization increases as it warms, so neutral water at 60 °C has pH ≈ 6.6 even though it is neither acidic nor basic.
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Sources
- Chemistry 2e, §14.2 pH and pOH — OpenStax
- Chemistry 2e, §14.1 Brønsted-Lowry Acids and Bases — OpenStax
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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