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, NH₃ was 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^-9, KbNH₃= 1.8 x 10^-5]