Chemistry / Unit 10 Chem 1850 worked example, 60 to 90 seconds
Calculating Percent Ionization
The idea
Calculating percent ionization means finding [H3O+] at equilibrium, either from a measured pH or by solving the Ka expression with an ICE table, then dividing that equilibrium value by the acid's original initial concentration and multiplying the result by 100 to express it as a percentage rather than a bare fraction.
In practice
A 0.125 M solution of nitrous acid measures pH 2.09, so [H3O+] = 10⁻²·⁰⁹ ≈ 8.1 × 10⁻³ M. Dividing 8.1 × 10⁻³ M by the initial 0.125 M and multiplying by 100 gives a percent ionization near 6.5%, a small fraction typical of a weak acid at this concentration.
Where people slip
Dividing the equilibrium [H3O+] by the equilibrium, rather than the initial, acid concentration gives a number that is not percent ionization at all.
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