Associated with any weak acid or weak base is a Ka or Kb value respectively.
Concept #1: Understanding Ka and Kb Expressions.
Weak acids possess a Ka value less than 1, while weak bases possess a Kb value less than 1. The equilibrium expressions of Ka and Kb are the same as other equilibrium constants we’ve seen.
Concept #2: Understanding Ka and Kb.
The greater the Ka value then the stronger the acid, while the greater the Kb the stronger the base. Ka and Kb are connected by the following equation:
Practice: If the Kb of NH3 is 1.76 x 10-5, determine the acid dissociation constant of its conjugate acid.
Example #1: Knowing that HF has a higher Ka value than CH3COOH, determine, if possible, in which direction the following equilibrium lies.
HF (aq) + CH3COO – (aq) ⇌ F – (aq) + CH3COOH (aq)
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) + ClO2 – (aq) ⇌ CN – (aq) + HClO2 (aq)
The acid dissociation constant of HCN is 4.9 x 10-10 and the acid dissociation of HClO2 is 1.1 x 10-2.
HCN (aq) + H2O (aq) ⇌ CN – (aq) + H3O+ (aq)
HClO2 (aq) + H2O (aq) ⇌ ClO2 – (aq) + H3O+ (aq)
Example #3: Which of the following solutions will have the lowest pH?
Practice: Which Bronsted-Lowry base has the greatest concentration of hydroxide ions?
Practice: Which Bronsted-Lowry acid has the weakest conjugate base?