Percent Ionization of a Weak Acid

Ka tells you how a weak acid behaves in principle; percent ionization tells you what actually happened in the beaker. In 0.100 M acetic acid the equilibrium [H⁺] is 1.34 × 10⁻³ M, so only 1.34% of the molecules have given up a proton and 98.7% are still intact: a vivid picture of what "weak" means. It is also the number that justifies the x-is-small approximation, which is generally trusted below about 5% ionization.

The counterintuitive part is that percent ionization rises as you dilute. Because [H⁺] goes as the square root of concentration while C goes linearly, halving the concentration multiplies the fraction ionized by roughly √2. Dilute that acetic acid to 0.0010 M and it is about 13% ionized even though the solution is far less acidic in absolute terms. Ostwald's dilution law captures the same effect, and it is why a weak acid approaches complete dissociation in the limit of infinite dilution.