PREPARATION
 
  • 3 Different brands of soda pop in cans (for 1 class a six pack of each is enough)

  • · Pepsi
    · Mountain Dew
    · Diet Sprite
  • Prepare flat versions of each of these by opening one or two cans of each and pour into beakers and agitate or leave out for a couple of days.  Each group will need about 30 mL of each flat pop.  Can put some into small labeled beakers for each group.
  • 1 Bottle phenolphthalein for each group (~25 mL total)
  • 1 Dropper bottle 1.0 M NaOH for each group (~ 500 mL total)

  • -or- fill an Erlenmeyer with about 100 mL NaOH for each group and supply disposable pipets
  • 1 Gallon distilled water, fill dispensers for each group
  • 2 250 mL Erlenmeyers for each group
  • 2 100 mL beakers for each group, one for the flat pop and the other for the fizzy pop
  • 1 10 mL graduated cylinder for each group
  • White sheets of scrap paper for each group to place under their reaction vessel

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    BACKGROUND

    There are several ingredients in soda pop that contribute to its acidity.  Today we will perform a TITRATION laboratory to determine the difference in acidities in different types of pop.  A titration is an experimental technique which allows determination of acid concentration in a substance, by adding a known volume of base until a pink color appears in the reaction vessel.  The pink color appears due to an INDICATOR which is added to the substance being tested.  The indicator, in this case phenolphthalein, is colorless in acid and turns pink in the presence of base.  Since our pop is already acidic, we can add the indicator, then add base just until the color changes.  This is called the ENDPOINT.  This type of experiment is a very common analytical technique used in various fields of study.

    DIRECTIONS

    You will be testing 3 kinds of pop, each fresh and each flat, so there are 6 tests to do.  If you go past the endpoint redo that test.

    1.  Open the can of pop and carefully pour about 30mL into a small beaker, share can with the other group at your bench.

    ***NOTE***
    Pour the pop CAREFULLY!!  You DONíT want it to fizz!  If it fizzes and loses its carbonation it will be just like the flat pop.

    2.  From the pop you poured into the small beaker measure out 20.0mL into a graduated cylinder
    3.  Pour this 20.0 mL into the Erlenmeyer flask.
    4.  Using distilled water bottle rinse remainder of pop from graduated cylinder into the flask also.
    5.  Now add about 25 mL distilled water to the Erlenmeyer
    6.  If you are testing the Pepsi make 2 of these solutions, one to use as a comparison for color and the other to test.
    7.  Add 7 drops of indicator to the flask
    8.  Rinse sides of Erlenmeyer with distilled water
    9.  Place flask on sheet of white paper so you can see the color change more clearly
    10.  Add drop of NaOH (base) 1 at a time; keep track!
    11.  Swirl Erlenmeyer as you make additions
    12.  As soon as you notice a color change which remains after you swirl the solution you have reached the endpoint.
     

    DATA

     
    place Pepsi Flat Pepsi Diet Sprite Flat Diet Sprite Mountain Dew Flat Mountain Dew
    Number of drops of NaOH place place place place place place
    QUESTIONS

    1.  Which contains less acid, the fresh or flat pop?  How do you know?  Why do you think that is?
     
     
     

    2.  What are the possible ingredients in the pop that contribute to its acidity?
     
     
     

    3.  Phosphoric acid is an ingredient in Pepsi, the structural formula is:

    When this acid dissociates 3 H+ (protons) are released.  That is said to be 3 equivalents of
    acid for every one mole of phosphoric acid.  How many moles of base do you think you would
    have to add to neutralize this acid?

    1 H3PO4  +   ___ NaOH  3 H2O  +  1 Na3PO4

    Letís assume the pop only contains phosphoric acidÖIf you had to add 20 drops of the NaOH
    to neutralize the Pepsi, and there are 16 drops NaOH in 1 mL of NaOH, how many grams of
    phosphoric acid were in your 20.0 mL sample?
     
     
     
     
     
     

    DISPOSAL
     

  • The extra opened pop should go down the drain, the rest can be used for the next group or taken home and consumed.
  • The NaOH can be put into a labeled storage bottle and used for later labs.  The dropper bottles should then be cleaned.
  • The phenolphthalein should be saved for other labs
  • If disposable pipets were used they can be pitched.  Or, you can rinse them thoroughly (without soap) and save them to be used with NaOH again.
  • Student solutions can all go down the drain

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    Ginger Chateauneuf, 2000.
     
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