MHS AP Chemistry
Identification of an Unknown Polyprotic Acid

In this experiment, you will be applying concepts learned in chapters 15-17 (and earlier, I guess) to identify an unknown polyprotic acid by determining the Ka values and the molar mass.

  1. Measure between 0.100 and 0.200 g of the unknown polyprotic acid.
mass of weighing dish __________ ___
mass of weighing dish + acid __________ ___
mass of acid  __________ ___
  1. Dissolve the acid in about 50 mL of distilled water.
  2. Titrate with 0.1 N NaOH solution.
  3. Graph the titration curve, and determine the values of Ka for the acid.  Clearly indicate the lines you are using on your graph.  If a value is not present, leave it blank.
pH (=pKa) Ka
first equivalence ½ way point __________ __________
second equivalence ½ way point __________ __________
third equivalence ½ way point __________ __________
  1. Determine the number of moles of base required to reach the first equivalence point.

  2. How many equivalents of acid were neutralized as of the first equivalence point?  Remember what an "Equivalent of Acid" is!

  3. Use your results so far to calculate the molar mass of the acid.

  4. Use the table on page 1016 of your textbook to identify the polyprotic acid.  Compare the accepted values for the acid you choose with the experimental values you determined today.  If there is more than one possibility, show them.

[MHS AP chem page]