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