Practice: If the Kb of NH_{3} is 1.76 x 10^{-5}, determine the acid dissociation constant of its conjugate acid.

Associated with any weak acid or weak base is a K_{a} or K_{b} value respectively.

Concept #1: Understanding K_{a} and K_{b} Expressions.

Weak acids possess a K_{a} value less than 1, while weak bases possess a K_{b} value less than 1. The equilibrium expressions of K_{a} and K_{b} are the same as other equilibrium constants we’ve seen.

Concept #2: Understanding K_{a} and K_{b. }

The greater the K_{a} value then the stronger the acid, while the greater the K_{b} the stronger the base. K_{a} and K_{b} are connected by the following equation:

Practice: If the Kb of NH_{3} is 1.76 x 10^{-5}, determine the acid dissociation constant of its conjugate acid.

Example #1: Knowing that HF has a higher K_{a} value than CH_{3}COOH, determine, if possible, in which direction the following equilibrium lies.

HF (aq) + CH_{3}COO ** ^{–}** (aq) ⇌ F

a) Equilibrium lies to the left.

b) Equilibrium lies to the right.

c) Equilibrium is equal and balanced.

d) Not enough information given.

Example #2: What is the equilibrium constant for the following reaction and determine if reactants or products are favored.

HCN (aq) + ClO_{2} ** ^{–}** (aq) ⇌ CN

The acid dissociation constant of HCN is 4.9 x 10** ^{-10}** and the acid dissociation of HClO

HCN (aq) + H_{2}O (aq) ⇌ CN ** ^{–}** (aq) + H

HClO_{2} (aq) + H_{2}O (aq) ⇌ ClO_{2} ** ^{–}** (aq) + H

Example #3: Which of the following solutions will have the ** lowest pH**?

- 0.25 M HC
_{2}F_{3}O_{2} - 0.25 M HIO
_{4} - 0.25 M HC
_{3}H_{5}O_{3} - 0.25 M H
_{2}CO_{3} - 0.25 M HSeO
_{4}^{–}

Practice: Which Bronsted-Lowry base has the greatest concentration of hydroxide ions?

Practice: Which Bronsted-Lowry acid has the weakest conjugate base?