Chemistry 104 Study Groups

BUFFERS & TITRATIONS PROBLEM
ANSWER
 

Exercise

  Consider methylamine (CH3NH2 Kb = 4.4 x 10-4). What is the pH of a 0.20 M solution of methylamine? What is the pH of a solution which is 0.20 M in methylamine and 0.20 M in the methylamonium ion?

 

Answer

 

The pH of a 0.20 M solution of methylamine is 12.0

The pH of a solution that is simultaneously 0.20 M in methylamine and 0.20 M in methylamonium chloride is 10.6

 

Explanation

 
  1. Calculation of pH is a straightforward equilibrium calculation.
  2. "x" can generally be neglected in these calculations.
  3. The presence of methylamonium ion inhibits the dissociation of methylamine, resulting in a lower (less basic) pH.


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