Intermediate Topic 525 of 1102 Acids, Bases & Electrochemistry
Percent Ionization
The fraction of acid molecules ionized, which increases with dilution.
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In-depth explanation of Percent Ionization
Percent ionization is the concentration of ionized acid divided by the initial concentration, multiplied by 100. For a weak acid this percentage increases as the solution is diluted, even though the absolute hydrogen ion concentration falls, a result known as Ostwald's dilution law. Dilution shifts the equilibrium toward the side with more particles, so a larger fraction of the molecules ionizes. Percent ionization is therefore not a fixed property of an acid and must be stated together with the concentration. It is a useful way to compare acids at the same concentration.
Going deeper: Percent Ionization
Key ideas: Percent ionization = [H+] at equilibrium divided by initial concentration times 100. It rises as the solution is diluted: roughly proportional to the square root of Ka over C. For 0.10 M acetic acid it is about 1.3 percent, and for 0.0010 M about 12 percent.
Common mistakes and connections: Mistake: thinking a more dilute acid is less ionized or that percent is constant. Another is using the 5 percent rule to justify dropping x when the result exceeds 5 percent; then solve the quadratic. Le Chatelier explains the trend, and the common ion effect lowers ionization.
Visualization of Percent Ionization
Ostwald dilution: the percent of acetic acid ionized climbs as the solution is diluted, from 1.3 percent at 0.10 M to 4.2 percent at 0.010 M. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A 0.10 M acetic acid solution is about 1.3 percent ionized, while a 0.010 M solution is about 4.2 percent ionized.
Frequently asked questions about Percent Ionization
What is Percent Ionization?
The fraction of acid molecules ionized, which increases with dilution.
Is Percent Ionization a beginner, intermediate or advanced topic?
Percent Ionization is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Percent Ionization?
Study these earlier topics in Acids, Bases & Electrochemistry first: Weak Bases, Acid Dissociation Constant, Ka and Kb Relationship.
What comes after Percent Ionization?
The next topics in Acids, Bases & Electrochemistry are pH of Weak Acid Solutions and Polyprotic Acids.
Can I practise Percent Ionization with quizzes?
Yes. The interactive lesson includes a self-check question on Percent Ionization, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Percent Ionization fits in Acids, Bases & Electrochemistry
Percent Ionization is topic 15 of 105 in Acids, Bases & Electrochemistry.