CHM3120L

Introduction to Analytical Chemistry: Laboratory

Polymer End Group Analysis - The Determination of Average Molecular Weight: Report Template

For each standardization titration calculate and report the result as molarity of titratable hydrogens. Calculate and report the average molarity and the standard deviation.
Calculate and report for each aliquot of PEG, the average value of , and the standard deviation.

Some Notes on Calculations

This experiment provides a good example of the usefulness of a back titration. The reaction between PEG and PMDA is slow. It would not be practical to titrate PEG end groups directly with PMDA. Adding an excess of PMDA and waiting 30 minutes drive the reaction to completion. The excess PMDA (actually the excess reactive hydrogens of pyromellitic acid) is then titrated with standard base. This reaction is rapid and thus suited to a titration with a visual end point.
You have not been provided with acid dissociation constants for pyromellitic acid. How do you know if titration to a phenolphthalein end point involves reaction of 1, 2, 3 or 4 acid hydrogens? You do not know, but you do not need to know because of the way the standardization and determination are handled.