CHAPTER 7.1: Weak Acids and Weak Bases

Weak Acid

Example 1

Phenol, C6H5OH is a weak acid used as a general disinfectant and in the manufacture of plastics.

  • Calculate the pH and the concentration of all species present (H3O+, C6H5O-, C6H5OH and OH- ) in a 0.10 M aqueous solution of phenol. 
  • Calculate the percent dissociation.

[Ka  =  1.3 x 10-10 ] 

Example 2

The ionization of phenylacetic acid,C6H5CH2COOH is as follows :

            C6H5CH2COOH  +  H2O <--> H3O+  + C6H5CH2COO        Ka = 4.9 x 10-5

Calculate the concentration of C6H5CH2COO- ion in 0.186M solution of C6H5CH2COOH. What is the pH of this solution?

Weak Base

Example 3

Pyridine, C5H5N, is a weak base. Calculate the percentage ionization of pyridine if the initial concentration of pyridine is 5.00 x10-4M.


An amount of 0.01 mol of concentrated ammonia, NHwas added to the above pyridine solution to give a final volume of 500 mL. Calculate the concentrations of ammonium and pyridium ion after the addition of the ammonia.

[Kb C5H5N=1.52 x 10-9KbNH3= 1.8 x 10-5]