Chemistry 104 Study Groups

BUFFERS & TITRATIONS PROBLEM
ANSWER
 

Exercise

  Consider the titration 20.0 mL of a 0.20 M solution of ethylamine with 0.10 M HCl. Calculate:
  1. The pH before any titrant has been added.

  2. The pH after 10.0 mL of titrant has been added.

  3. The pH after 20.0 mL of titrant has been added.

  4. The pH at the stoichiometric point.

  5. The pH when 2.0 mL of titrant has been added, past the stoichiometric point.

  6. What volume corresponds to a position half-way to the stoichiometric point? What is the significance of this point?

 

Answer

 

  1. The pH before any titrant have been added is 12.0
  2. The pH after 10.0 mL of titrant have been added is 11.2
  3. The pH after 20.0 mL of titrant have been added is 10.6
  4. The pH at the stoichiometric point is 5.9 (The stoichiometric point occurs when 40.0 mL of titrant have been added.)
  5. The pH when t.0 mL of titrant has been added, past the stoichiometric point is 2.5
  6. When 20.0 mL of titrant have been added, the point half-way to the stoichiometric point has been reached. At this point pH = 14 - pKb

 

Explanation

 
  1. Examination of the expression for Kb reveals that the desired ratio can be obtained by manipulating the expression.
  2. Be sure that the correct ratio is calculated, the methylamine concentration must be in the numerator.


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