The acid dissociation constant, Ka, and the base dissociation constant, Kb, determine the strengths of weak acids and weak bases respectively. 

Ka and Kb

Example #1: Associated with any weak acid or weak base is a Ka or Kb value respectively. 

Ka examines the strength of weak acids. 

Kb examines the strength of weak bases. 

Example #2: Consider two aqueous solutions of equal concentration. Which statement is true?

chlorous acid (HClO2, Ka = 1.1 x10 -2 ) and phenol (HC6H5O, Ka = 1.3 x10 -10)

a) HClO2 produces more [H3O+] than HC6H5O

b) HClO2 is basic compared with HC6H5O

c) HClO2 produces less [H3O+ ] than HC6H5O

d) HClO2 is a strong acid

e) ClO2– produces more [OH] than C6H5O

Ka and Kb Calculations

Example #3: Which of the following compounds has the strongest conjugate acid?

a) C2H5NH2        (Kb = 5.6 x 10-4)

b) H2NNH­2         (Kb =  1.3 x 10-6)

c)  NH3               (Kb =  1.75 x 10-5)

d)  HONH­2          (Kb = 1.1 x 10-8)

Example #4: At 0 oC, the ion product constant of water is 1.2x10–15. The pH of pure water at this temperature is:

a) 6.88

b) 7.00

c) 7.46

d) 7.56

Practice: A − is a weak base. Which equilibrium corresponds to the equilibrium constant Ka for HA?