This relatively brief but fairly difficult lesson applies the concepts of the previous lesson (Calculations 1: Formulas and Moles) to the determination of the empirical formula by combustion analysis. This is the actual procedure used in research laboratories to help verify the composition of newly synthesized compounds. It starts with an animation of the combustion process. A weighed sample of the unknown material is burned in a stream of oxygen. The products, CO2 and water, are collected in appropriate traps which are weighed before and after.
The student is guided through a calculation of the formula of a hydrocarbon. The masses of the traps are given before and after the combustion.
The student is invited to deduce the moles of CO2 and water produced from the masses collected, then use these to infer the moles of carbon and hydrogen that were in the compound that was burned, and thus the empirical formula.
Hints and helpful feedback are given at each step, and if the student requests it or shows by incorrect answers that he needs it, the procedure is presented in detail.
As shown in the figure, a table of useful data and a calculator are provided on the screen, though the student is encouraged to use his/her own calculator as well as pencil and paper.
The next two chapters similarly show how to deal with compounds that also contain oxygen or nitrogen. The mass of nitrogen or oxygen must be inferred by difference, thus yielding the moles of that element. There is a guided calculation, with lots of helpful feedback, of the formula of a compound that contains such a third element.
The lesson ends with a quiz, in which combustion is simulated. The masses of the traps before and after the combustion are given; the student is required to produce the formula. There is a large variety of compounds in the dataset. The student must try at least five of these problems to produce a score, which is recorded in a dataset. The top scores (up to 15) and a histogram of all scores on this quiz may be viewed in the final chapter.
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Updated July 18, 2000