Chemistry 104 Study Groups

BUFFERS & TITRATIONS PROBLEM
ANSWER
 

Exercise

  Calculate the pH of the solution in 2a after 0.1 g NaOH has been added (assume no volume change). Calculate the pH of the solution in 2b after the same amount of strong base has been added.

 

Answer

 

The pH of buffer a (initial pH = 10.0) remains 10.0

The pH of buffer b (initial pH = 11.3) rises very slightly to 11.4.

 

Explanation

 
  1. Calculate the moles of base (methylamine) and acid (methylamonium ion) present in each buffer solution.
  2. Caculate the moles of added OH-
  3. The methylamonium ion and the OH- will react in a 1:1 stoichiometry.
  4. An amount of methylamonium ion equal to the number of moles of added OH- will be converted to methylamine.
  5. Calculate the new concentrations of methylamine and methylamonium ion using the new quantities.
  6. Calculate the pH using the expression for Kb


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