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?