Percent Ionization Calculator
Find what fraction of a weak acid is ionized at equilibrium from its Ka and concentration — the degree of dissociation as a percentage.
How it works
% ionization = ( [H⁺] / C₀ ) × 100 , with [H⁺] = ( −Ka + √(Ka² + 4·Ka·C₀) ) / 2
Percent ionization is the fraction of weak-acid molecules that have donated their proton at equilibrium, expressed as a percentage of the starting concentration. It rises as the solution is diluted — a counter-intuitive result of Le Chatelier’s principle — even though the actual [H⁺] falls. The calculator solves the equilibrium quadratic first, then divides the ionized amount by the initial concentration.
Worked example
For 0.10 M acetic acid (Ka = 1.8 × 10⁻⁵), the equilibrium [H⁺] is 1.33 × 10⁻³ M. Percent ionization = (1.33 × 10⁻³ / 0.10) × 100 = 1.33 %. Dilute the same acid to 0.010 M and the percent ionization rises to about 4.2 %.
Frequently asked questions
Why does percent ionization increase on dilution?
Diluting shifts the ionization equilibrium toward more dissociation (Le Chatelier), so a larger fraction ionizes — even though the absolute [H⁺] and thus acidity decrease.
Is percent ionization the same as the degree of dissociation?
Yes. The degree of dissociation α is the fraction ionized; multiply it by 100 to get percent ionization.
How does percent ionization relate to acid strength?
At a given concentration, a stronger acid (larger Ka) has a higher percent ionization. Strong acids approach 100 %.
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This calculator is for educational purposes. Double-check important results before acting on them.