In this in-class activity, each student calculates the inital pH, equivalence volume, and pH at the equivalence point for both a strong acid-strong base and a weak acid-strong base titration.
In addition, each student is assigned a unique volume before the equivalence point and a unique volume after the equivalence point for each titration curve.
The data from the class is then assembled in Excel to construct the two titration curves.
This forces each student to do the calculations for each of the four regions of both types of titration curves. This activity could be used to introduce titration curves or to reinforce previously covered lecture material/problem-solving. It could also be switched to do a strong base-strong acid or a weak base-strong acid titration curve.
The constructed titration curves can be used for further discussions of the differences between a strong acid and a weak acid in terms of initial pH, the rapid-rise portion of the curve, and the pH at the equivalence point.
A student should be able to
- determine the pH of a strong acid solution
- determine the pH of a weak acid solution using Ka
- use stoichiometry to calculate equivalence volumes for acid-base titrations
- employ limiting reagent calculations to determine acid or base concentrations for different regions of a titration curve and determine pH
- determine the pH of a weak base solution using Ka, Kb
notecards with assigned volumes
computer for entering volume and pH data
This could be done as an in-class activity (I used a 3 hr lab period - most students took less than 2 hrs) or as a take-home assignment. Students were allowed to use their notes and textbooks. I did not strictly forbid them from working together, but I did tell them that I wanted them to be sure that they could do all of the calculations themselves.
I had an Excel spreadsheet of the correct pH values for each volume (attached). Students were allowed to come check their work with me and continue working if their answers were incorrect. I was also able to help them if they got stuck.
Attached are the student worksheets, the class titration curves, and the Excel file I used to calculate the correct pH values. I chose volumes and molarities that would give me an appropriate number of volumes before the equivalence point. Volumes and molarities should be adjusted as needed for the size of your class.
I used whole number volumes, but I think it would be better to have smaller volume increments near the rapid-rise portions of the curves so it doesn't look like the data "jumps" as much.
Students were allowed to keep working until they had correct pH values, so they were graded on participation. Worksheets were collected at the end in order to construct the titration curve.
This could be collected and graded for correctness.
Students were evaluated on similar questions on the subsequent exam. Most students (12 out of 15) scored 11-13 points on a 13 point question where they had to solve for the pH in the four regions of a strong acid titration curve. 8 out of 15 recieved full credit on a question where they had to calculate the pH in the buffer region of a weak acid titration curve.