Read Ionic Equilibrium as a pH-Tracking Exercise
Ionic equilibrium questions almost always reduce to tracking one quantity — [H⁺] — through a dilution, a mixture, or a solubility condition, then converting to pH at the very last step.
Ionic Equilibrium & pH Calculations is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
pH after dilution of a strong acid
Diluting a strong acid dilutes [H⁺] proportionally (moles conserved, volume increases) — recompute [H⁺] from the new volume before taking -log, rather than trying to shortcut the pH change directly.
Mixing acids and bases: net moles decide everything
When a strong acid and strong base are mixed, subtract the smaller mole count from the larger to find the net excess of H⁺ or OH⁻, then divide by the total combined volume — the identity of whichever species is in excess sets whether you compute pH or pOH first.
Relative acid strength from conjugate base strength
A stronger acid has a weaker conjugate base, and vice versa — use this inverse relationship to rank acid strength when only conjugate-base behaviour is given, without needing explicit Ka values.
Solubility product and maximum pH
A maximum pH before a hydroxide precipitates comes directly from Ksp: at the threshold, [M ion][OH⁻]^n = Ksp exactly, so solve for [OH⁻] at that boundary and convert to pH.
OlympIQ exam lens
- ✓Dilution of a strong acid: recompute [H⁺] from conserved moles over the new volume, then take -log.
- ✓Mixing strong acid and base: net moles of excess H⁺ or OH⁻ over total volume, not average pH.
- ✓Stronger acid ⇔ weaker conjugate base — use this to rank strength without Ka values.
- ✓Maximum pH before precipitation: solve Ksp = [ion][OH⁻]^n for [OH⁻] at the threshold.
Don't average pH values when mixing solutions — pH is logarithmic, so you must convert to [H⁺] or [OH⁻], combine those linearly by moles and volume, and only then convert back to pH.
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