Percent Ionization of a Weak Acid
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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.
- = Percent ionization
- = Hydrogen ion concentration at equilibrium
- = Formal acid concentration
- Percent ionization — Percent Yield, Mass Percent of a Solution
- Hydrogen ion concentration at equilibrium — pH from Hydrogen Ion Concentration, Molarity (C = n/V)
- Formal acid concentration — pH of a Weak Acid from Ka, Henderson–Hasselbalch Equation (Weak Acid Buffer)