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Download: antacid_prelab.docx

Downloading: antacid_prelab.docx How to run a titration lab without marking any papers

Acid-base titration of commercial antacids is a common high school chemistry lab exercise. Students obtain an over-the-counter antacid (in this case, calcium carbonate), calculate the volume of 0.5 M hydrochloric acid necessary to neutralize it, titrate it, and calculate results and yield. Many undergraduate chemistry lab manuals suggest a back titration with a strong base because calcium carbonate is insoluble, but with careful technique, an acid titration can yield consistent results within 1% of the labelled active ingredient, which I consider fully adequate for the purposes of high school general chemistry. This also provides a natural opportunity to revisit earlier topics like solubility rules and types of mixtures.

In this exercise, students choose an antacid with active ingredient calcium carbonate, magnesium hydroxide, or any combination of these, and titrate it using 0.5 M HCl and an appropriate pH indicator. They complete a pre-lab worksheet where the expected volume of HCl is calculated stoichiometrically, and then perform several titrations (preferably 6, but no less than 3). The three measurements closest to each other are then averaged and used to calculate the percent yield.

The following data is then entered online into Moodle, and the computer checks their calculations. The Multinumerical question type plugin must be installed on the server and a quiz must be created using the attached item at the bottom of this post.

  1. How many milligrams of calcium carbonate does one dose contain? If none, enter 0. (prelab #2)
  2. How many moles of calcium carbonate is that? If none, enter 0. (prelab #3)
  3. How many milligrams of magnesium hydroxide does one dose contain? If none, enter 0. (prelab #2)
  4. How many moles of magnesium hydroxide is that? If none, enter 0. (prelab #3)
  5. How many TOTAL moles of HCl are theoretically needed to neutralize ALL active ingredients in one dose? (prelab #5)
  6. How many mL of 0.50 M HCl would contain that many moles? (prelab #6)
  7. "Middle" HCl used, in mL (Analysis #1)
  8. "High" HCl used, in mL (Analysis #1)
  9. "Low" HCl used, in mL (Analysis #1)
  10. Average volume of HCl used (Analysis #2)
  11. Moles of HCl actually used (Analysis #3)
  12. Percent yield (actual moles HCl ÷ theoretical moles HCl)

The computer will then perform the following checks on each student entry, and award one point for each answer that's validated:

  1. Is the number of mg calcium carbonate realistic for an over-the-counter antacid?
  2. Was this entry (whatever it was) converted correctly to moles?
  3. Is the number of mg magnesium hydroxide realistic for an over-the-counter antacid?
  4. Was this entry (whatever it was) converted correctly to moles?
  5. Should be equal to twice the sum of #2 and #4 (as both possible active ingredients have 2:1 stoichiometry with HCl), allowing a small margin for rounding errors
  6. Applies the molarity formula to see if student correctly converted mol HCl to mL HCl
  7. Checks to see how close the middle titration outcome was to the expected volume; if within 5%, awards a point for accuracy
  8. Checks to see how close the high titration outcome was to the middle; if within 1%, awards a point for precision
  9. Checks to see how close the high titration outcome was to the middle; if within 1%, awards a point for precision
  10. Did the student correctly calculate the mean of their three trials (allowing a small margin for rounding errors)?
  11. Did the student use the average in #10 (regardless of whether it was calculated correctly) to calculate actual moles of HCl?
  12. Was percent yield calculated correctly based on student entries for #5 and #11 (even if these entries were themselves incorrect)?

Here is a sample of feedback provided after a typical student submission:

How to run a titration lab without marking any papers

Students are allowed to redo their calculations and resubmit for full credit, or even to redo the experiment and collect entirely new data.

Credit to Robert Farber's "Off the Shelf Chemistry" for the original lab concept and to Nicolas Dunand for the Moodle plugin.