Calculator
Choose one known value. This educational tool uses pH + pOH = 14 at 25 °C.
Neutral
Formula and assumptions
pH = −log₁₀[H⁺]; pH + pOH = 14
The tool derives the other displayed values from one entered value using the common aqueous 25 °C relationship. In rigorous chemistry, pH is defined using hydrogen-ion activity rather than concentration.
Assumption: The common aqueous relationship pH + pOH = 14 is used at 25 C.
Variables and units
Worked example
For [H⁺] = 1 × 10⁻⁷ mol/L, pH = 7.00 in this idealised calculation.
pH, [H⁺] and familiar liquids
| [H⁺] (mol/L) | pH | pOH at 25 °C |
|---|---|---|
| 1e-1 | 1 | 13 |
| 1e-2 | 2 | 12 |
| 1e-3 | 3 | 11 |
| 1e-7 | 7 | 7 |
| 1e-11 | 11 | 3 |
| 1e-13 | 13 | 1 |
Each pH unit is a tenfold change in hydrogen-ion concentration, so pH 3 is ten times more acidic than pH 4. A 0.01 M solution of a strong acid such as HCl dissociates completely and has pH 2; a weak acid at the same concentration has a higher pH because only part of it dissociates. Approximate values: lemon juice about 2, pure water 7 at 25 °C, seawater about 8, household ammonia about 11. The relationship pH + pOH = 14 holds at 25 °C; at other temperatures the sum changes slightly.
Important limits
Rigorous pH is based on activity, not simply concentration; this is not a buffer or equilibrium model.
Reference and related tools
IUPAC: pH. Return to Chemistry Calculators for the other tools in this group.