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Year 12 / pH, pOH and Kw

One step. Ten times.

pH is a logarithmic measure: pH = −log₁₀[H+H^{+}]. A decrease of one pH unit means ten times the hydrogen ion concentration. Concentration changes are multiplicative, not additive.

At 25 °C, Kw = [H+H^{+}][OH⁻] = 1.0 × 10−1410^{-14}, so pH + pOH = 14.00. This numerical total is temperature-dependent. For a strong monoprotic acid, [H+H^{+}] is approximately the acid concentration when water’s contribution is negligible.

For a fully dissociated base, use its formula: 0.010 M Ba(OH)₂ supplies 0.020 M OH⁻. Extremely dilute solutions need the contribution from water and are outside the simple model shown here.

INTERACTIVE MODEL

Strong is not concentrated

H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺H⁺HA: undissociated · H⁺ and A⁻: dissociatedpH 2.00014
c = 0.010 M · pH 2.00 · ionised ≈ 100.0%

At 25 °C. Compare HCl with a model weak acid (Ka = 1.8 × 10⁻⁵). The pH calculation includes water; particle counts are schematic.

Explore: Change the hydrogen-ion concentration and connect each tenfold change with pH.

pH = −log₁₀[H⁺] pOH = −log₁₀[OH⁻]
WORKED EXAMPLE

Find the pH of 0.0020 mol L−1L^{-1} HCl at 25 °C.

  1. pH = −log₁₀(0.0020).
  2. pH = 2.70.

Answer: 2.699 pH

Assumed knowledge

Calculator log and 10ˣ functions; full dissociation for the strong species stated.

Learning checkpoints & sourceYOUR SYLLABUS CHECKPOINT
  • Calculate pH and pOH of strong acids and bases.
  • Use Kw to relate hydrogen and hydroxide ions.
QCAA Chemistry 2025 v1.3 · p. 33 ↗
READY TO TRY IT?

Turn the idea into an answer.

Back to syllabusNext: Ka, Kb and pKa
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