pH and pOH: Formulas and Worked Examples
Use the pH and pOH formulas to classify acids and bases, with two worked examples.
The formulas
- pH = −log[H⁺]
- pOH = −log[OH⁻]
- pH + pOH = 14 (at 25 °C)
pH below 7 is acidic, 7 is neutral, above 7 is basic.
Example 1
Find the pH when [H⁺] = 1 × 10⁻³ M.
- pH = −log(10⁻³)
- pH = 3
pH = 3 (acidic)
Example 2
A solution has pOH = 4. Find its pH.
- pH = 14 − pOH
- pH = 14 − 4 = 10
pH = 10 (basic)
Common mistake
Thinking pH 3 is only slightly more acidic than pH 4
The pH scale is logarithmic: each step is a 10× change in [H⁺].
Lower pH means more acidic, not less.
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