Lecture Notes from CHM 1341
26 June 1996

Stoichiometry

Stoichiometry is the art and science of balancing chemical equations. As such, it is critical to the predictions of quantities of reactants and products because it gives their proportionality factors, viz., the ratio of their stoichiometric coefficients, those multipliers before the substance symbols indicating the numbers of molecules (or moles) taking part in the reaction. From those proportions, one can easily convert forwards and backwards not only the moles of reactants and products but also their weights, (ideal) gas volumes, heats absorbed or evolved, electric currents flowing, indeed any extensive quantity (that is, any value associated with the reaction which changes as the scale of the reaction changes).

In doing all these clever conversions, it is important (tedious but important) to LABEL ALL THE FACTORS involved. While I concede that this takes more time and space on your paper, it has the marvelous advantage of letting you know when you've inadvertantly inverted some proportionality! Dimensional analysis should not be sneezed at; those fortuitous units cancellations build confidence in the viability of your answers.

Another terribly handy property of stoichiometry lies not just in quantitative analysis, but in its utility to establish molecular formulae from decomposition data. This use suggests that it is important to use molar weights accurate to 2 decimal places. If you're cavalier about significance when dealing with reasonably sized molecules, you might miss or overcount light weight things like hydrogen atoms. Those errors may cause you to predict incorrect bonding (since you can imagine that too few hydrogens in a hydrocarbon, for example, is evidence of carbon-carbon multiple bonding).

While the folks who used to do quantitative analysis, Analytic Chemists, were once sneered at as the mere "accountants" of Chemistry, they are now in high demand as the developers of sophisticated instrumentation to quantify samples all the way down to the individual atom level. (One experiment has held a single ion in place in a vacuum while probing it with light and collecting its individual emissions. Amazing!) Our own Inga Musselman, for instance, uses Atomic Force Microscopy to observe the morphology of individual molecules on surfaces; her current interests include abnormal fibers in the brains of Alzheimer's patients. She's asking the dual questions all analytic chemists ask: What's there and how much?

In fact, the first question (which is called "qualitative analysis") has to be answered before the second is asked, because the techniques of analytic chemistry are sensitive to contaminants; these must be counted and/or eliminated before analysis of the targets is begun.

Chris Parr
University of Texas at Dallas
Programs in Chemistry, Room BE3.506
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Richardson, TX  75083-0688

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