Intermediate Topic 524 of 1102 Acids, Bases & Electrochemistry
Ka and Kb Relationship
For a conjugate pair, Ka times Kb equals Kw.
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In-depth explanation of Ka and Kb Relationship
For any conjugate acid-base pair, the product of the acid constant and the base constant equals the ion product of water: Ka x Kb = Kw. Taking negative logarithms gives pKa + pKb = pKw, which is 14.00 at 25 C. This means that if one constant is known, the other can be found without a separate measurement. A strong acid has a very large Ka, so its conjugate base has a tiny Kb, and the reverse is also true. The relationship is used constantly when working with salts and buffers made from weak acids or bases.
Going deeper: Ka and Kb Relationship
Key ideas: For a conjugate pair, Ka x Kb = Kw, equivalently pKa + pKb = 14.00 at 25 C. Given Ka of HCN (6.2e-10) you obtain Kb of CN- as 1.6e-5. This lets you treat salt hydrolysis with one table of Ka values.
Common mistakes and connections: Students pair the wrong species: the Kb belongs to the conjugate base of that particular acid, not to any base in the solution. Also do not apply 14 at other temperatures; use that temperature's Kw. A weaker acid has a stronger conjugate base, which explains why NaCN solutions are basic.
Visualization of Ka and Kb Relationship
pKa + pKb = 14 for a conjugate pair, so a stronger acid (small pKa) has a very weak conjugate base (large pKb). The interactive lesson shows this as a live animation or chart.
Worked example and use case
The Ka of NH4+ equals 1.0 x 10^-14 divided by the Kb of NH3, 1.8 x 10^-5, giving about 5.6 x 10^-10.
Frequently asked questions about Ka and Kb Relationship
What is Ka and Kb Relationship?
For a conjugate pair, Ka times Kb equals Kw.
Is Ka and Kb Relationship a beginner, intermediate or advanced topic?
Ka and Kb Relationship is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Ka and Kb Relationship?
Study these earlier topics in Acids, Bases & Electrochemistry first: Weak Acids, Weak Bases, Acid Dissociation Constant.
What comes after Ka and Kb Relationship?
The next topics in Acids, Bases & Electrochemistry are Percent Ionization and pH of Weak Acid Solutions.
Can I practise Ka and Kb Relationship with quizzes?
Yes. The interactive lesson includes a self-check question on Ka and Kb Relationship, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Ka and Kb Relationship fits in Acids, Bases & Electrochemistry
Ka and Kb Relationship is topic 14 of 105 in Acids, Bases & Electrochemistry.