Ka and Kb

What are Ka and Kb?

When predicting whether a species would donate or take protons at a given pH value, K_a, pK_a, K_b, and pK_b are the most useful parameters.

The dissociation constants for acids and bases are commonly stated in moles per litre (mol/L). The acid dissociation constant is K_a. The -log of this constant is basically pK_a. The base dissociation constant is K_b, while the -log of the constant is pK_b.

The acid dissociation constant K_a and the base ionisation constant K_b for conjugate-acid base pairings are related by the formulae below:

K_a﹒K_b = Kw, where K_w  is the autoionisation constant.

⇒ pK_a + pK_b = 14 at 25℃

Table of Contents

• Introduction: Ionization Of Weak Acids And Bases Is Reversible
• For HA Reacting As An Acid, We Need To Find Ka
• Finding Kb For A^– Reacting As A Base
• Relationship Between Ka And Kb For Conjugate Acid-Base Pair
  • What is Kw?
• Frequently Asked Questions

Introduction: Ionization Of Weak Acids And Bases Is Reversible

Weak acids, abbreviated as HA, donate hydrogen (or proton) to water, forming the conjugate bases A^– and H₃O⁺, as shown below:

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A^–(aq)

Similarly, in water, a base (abbreviated as B) accepts a proton to generate the conjugate acid, HB⁺ and OH^–, as shown:

B(aq) + H₂O(l) ⇌ HB⁺(aq) + OH^–(aq)

The ionisation reaction’s equilibrium constant quantifies the relative amounts of each species given a weak acid or base. The relationship between the equilibrium constants Ka and Kb for a conjugate acid-base pair will be discussed in this article.

NOTE: All solutions will be assumed to be aqueous solutions for the purposes of this article.

For HA Reacting As An Acid, We Need To Find Ka

Taking a close look at the dissociation reaction for a monoprotic weak acid HA:

A-, the conjugate base of HA, and H₃O⁺ result from this reversible reaction. For the equilibrium constant K_a, we can construct the following expression:

Read More:

What is Ka in Chemistry?

Finding Kb For A- Reacting As A Base

We may alternatively express the reversible process for A^– acting as a base by accepting a proton from water because A^– is a base:

HA and OH^– are the end products of this reaction. The equilibrium constant K_b for the reaction where A^– functions as a base can be written as follows:

While this appears to be the inverse of HA acting as an acid, the two reactions are quite distinct. H₃O⁺ is one of the results of HA serving as an acid. One of the conjugate base A- acting as a base is OH^–.

Read More:

What is the Kb Formula?

Relationship Between Ka And Kb For Conjugate Acid-Base Pair

When K_a for HA is multiplied by the K_b of its conjugate base A-, we get:

where K_w is the water dissociation constant.

What is Kw?

K_w is the equilibrium constant for the reversible process that produces H₃O⁺ and OH^– from two water molecules.

H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH^–(aq)