Example 1
A 0.100 mol L−1 monoprotic acid has [H+]=0.00400 mol L−1. Neglect water. Find percentage ionisation.
Ionisation =c[H+]×100=0.1000.00400×100=4.00%.
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Year 11 · SACE Stage 1
Acid strength and ionisation
Distinguish strength from concentration and calculate ionisation amounts.
Method and worked examples
Acid strength describes the extent of ionisation in water. A strong acid is effectively completely ionised in the dilute solutions used here, while a weak acid is only partly ionised. Concentration measures total solute per volume, so a weak acid can be concentrated and a strong acid dilute. At the same concentration, different ionisation fractions can give different hydrogen-ion concentrations and conductivities.
For a monoprotic acid with ionised fraction f, and negligible contribution from water, [H+]= fc where c is total acid concentration. Convert percentages to fractions before multiplying. Reverse this relation to calculate percentage ionisation as c[H+]×100. Compare strengths only under stated comparable conditions: measured [H+] alone cannot distinguish a dilute strong acid from a more concentrated weak acid.
Example 1
Example 2
A 0.100 mol L−1 monoprotic acid is 2.0% ionised. Neglect water autoionisation. Find [H+].
[H+]=0.100×0.020=0.00200 mol L−1. Total acid concentration and hydrogen-ion concentration are different.
Watch out: Strength is extent of ionisation; concentration is amount per solution volume.