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Acid-Base Titrations (Mullaugh)

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    Learning Objectives

    Following this activity, students should be able to:

    • Determine the pH at any point throughout an acid-base titration including the titration of:
      • A strong acid with a strong base
      • A strong base with a strong acid
      • A weak acid with a strong base
      • A weak base with a strong acid
    • Describe the general nature of the solution in terms of how to calculate pH for the following regions of an acid-base titration:
      • Region I: before any titrant added
      • Region II: partway to the equivalence point
      • Region III: at the equivalence point
      • Region IV: past the equivalence point
    • Compare how concentration and strength of an acid or base changes the appearance of a titration curve.

    Consider the following titration…

    What is the pH at the following points described for the titration of 75.0 mL of 0.0874 M trimethylamine, (CH3)3N, with 0.250 M HBr. Trimethylammonium, (CH3)3NH+, has a pKa = 9.80.

    1. Circle one from each four pairs of words. The titration described is the titration of a (weak/strong) (acid/base) with a (weak/strong) (acid/base).

     

    1. Write the overall reaction between the analyte and the titrant. What is the titrant and what is the analyte in this titration?

     

     

     

    1. What is K for the reaction as written? What does the magnitude of K tell you about the reaction?

     

     

     

     

     

    1. What is the initial pH of the (CH3)3N solution before any HBr is added? (Region I)

     

     

     

     

     

     

     

    1. What is the pH after the addition of 10.0 mL 0.250 M HBr? (Region II)

     

     

     

     

     

     

     

     

    1. What is the equivalence point (Ve)? What is the pH at the equivalence point? (Region III)

     

     

     

     

     

     

     

     

     

     

    1. What is the pH after 35.0 mL HBr is added? (Region IV)

     

     

     

     

     

     

     

     

     

     

    1. How would you describe the solution at each of the previous four point in the titration?

      Question

      Region

      HBr added*

      The solution is a…

      #4

      I

      V = 0 mL

       

      #5

      II

      V < Ve

       

      #6

      III

      V = Ve

       

      #7

      IV

      V > Ve

       

      *Volume (V) of titrant added with Ve = equivalence point.

     

    1. Using your calculations above, sketch what the titration curve would look like. Label the equivalence point.

    Graph_TitrationCurve.png

    1. At what point is the pH equal to the pKa of (CH3)3NH+? Label this point on the graph above.

     

     

    1. Discuss how your group how the titration curve above would differ if the following changes were made:
      1. The initial concentration of trimethylamine was lower.

         

         

      2. Instead of trimethylamine, potassium hydroxide (KOH) was the analyte.

         

         

         

      3. Instead of trimethylamine, analine was the analyte. The conjugate acid of analine has a pKa = 4.601.

     

     

     

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