Example 1 · level 1
What is the pH of a solution of HNO₃ of concentration 1 × 10⁻⁴ mol/L at 25 °C? (It is a strong acid and ionises completely.)
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- [H⁺] = 10⁻⁴ mol/L.
- pH = −log(10⁻⁴) = 4.
Answer: 4
CLASS 11–12 CHEMISTRY · EQUILIBRIUM
pH of strong acids and bases
pH = −log[H⁺]. For a base, find pOH = −log[OH⁻], then pH = 14 − pOH (at 25 °C). Strong acids and bases ionise completely.
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Worked examples
Example 2 · level 2
Calculate the pH of 5 × 10⁻² M NaOH at 25 °C, correct to 2 decimal places. Assume complete ionisation. (log 2 = 0.3010, log 3 = 0.4771, log 5 = 0.6990)
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- [OH⁻] = 5 × 10⁻² mol/L.
- pOH = −log(5 × 10⁻²) = 2 − log 5 = 1.301.
- pH = 14 − 1.301 ≈ 12.7.
Answer: 12.7
Example 3 · level 3
Calculate the pH of 1.5 × 10⁻³ M H₂SO₄ at 25 °C, correct to 2 decimal places. Assume complete ionisation. (log 2 = 0.3010, log 3 = 0.4771, log 5 = 0.6990)
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- Each formula unit gives 2 H⁺, so [H⁺] = 2 × 1.5 × 10⁻³ = 3 × 10⁻³ mol/L.
- pH = −log(3 × 10⁻³) = 3 − log 3 = 2.5229.
- pH ≈ 2.52.
Answer: 2.52