Chemical Education Today
Commentary
Mole City: A Stoichiometric Analogy by Addison Ault
Reading the recent paper by Cook and Cook (1), I found myself in complete agreement with their goals, which include getting students to “…first understand the problem they are trying to solve…” and getting them to “…draw a mental map with clear directions toward a solution…”. As I pondered these things I slipped into a reverie in which I had a vision of a city. Allow me to describe my vision of what I shall call Mole City. Mole City The main street of Mole City, call it Mole Street, runs east and west. It is cut by the river, which runs north and south. To the left of the river live the Starting Materials; to the right of the river live the Products. The houses of the Starting Materials lie on east–west streets parallel to Mole Street—streets named Gram Street, Volume of a Pure Solid Street, Volume of a Pure Liquid Street, and Volume of a Pure Gas at STP Street. Farther out can be found Volume of a Gas at T and P Street, and Volume of a Solution Street. It is, however, only Mole Street that has a bridge, the Mole Street Bridge, that crosses the river. All the other streets are cut by the river. The houses of the Products also lie on these east–west streets, parallel to Mole Street. In contrast to the houses of the Starting Materials, however, the houses of the Products are on those parts of these streets that are to the right of the river. Again, the streets are Gram Street, Volume of a Pure Solid Street, Volume of a Pure Liquid Street, Volume of a Pure Gas at STP Street, Volume of a Pure Gas at T and P Street, and Volume of a Solution Street. On this right side of the river there is also Percent Yield Street. When Starting Materials are looking for action they move from their homes to Mole Street. To do this they travel on the avenues, which run from north to south. Each north– south avenue has a sign that indicates the Conversion Factor that is required for passage toward or away from Mole Street. For the streets whose names are mentioned above, these Conversion Factors are Molar Mass Avenue, Density of a Pure Liquid Avenue, Density of a Pure Solid Avenue, Molar Volume of a Pure Gas at STP Avenue, N ⫽ PV/RT Avenue, and Moles per Liter of Solution Avenue. Once on Mole Street and properly quantified by Mole Number the Starting Materials head to the right, toward the river. Starting Materials that live on Mole Street, can, of course, move directly toward the river.
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It is in the reactors that line the Mole Street Bridge, which lies across the river, where the action is to be found. Some reactors, often called “bars”, feature acid, some base, some heavy metal, some a solvent such as ethanol (or, more rarely, water), but each provides a place where old connections can be overcome and new hookups achieved. The Products thus formed on the bridge eventually leave and then move further to the right on Mole Street, turning onto the avenues to reach their desired destinations on Gram Street, Volume of a Pure Solid Street, Volume of a Pure Liquid Street, Volume of a Pure Gas at STP Street, Volume of a Gas at T and P Street, or Volume of a Solution Street. When, occasionally, some Product is lost the final destination will be a house on Percent Yield Street. There will, of course, be times when the “chemistry” in the bar is not right and some Starting Materials will be left over after the new relationships are established. The leftovers will then have to go back to the left on Mole Street, to the shelters on Moles Remaining Street. I describe the construction and use of such “maps” in Part 2 of my recent paper (2). Comments We all try to teach our students how to think; that’s what teaching is about. I also believe that a student is more likely to think when the logical steps are clearly laid out, as, for example, paths on a map. As teachers it is our duty to show our students how to travel efficiently over the best paths. And these may not be the paths we learned to travel when we were students. The creativity of teaching is in the discovery of better paths. Literature Cited 1. Cook, E.; Cook, R. L. Cross-Proportions: A Conceptual Method for Developing Quantitative Problem-Solving Skills. J. Chem. Educ. 2005, 82, 1187–1189. 2. Ault, A. How To Say How Much: Amounts and Stoichiometry. J. Chem. Educ. 2001, 78, 1347–1349.
Addison Ault is a member of the Department of Chemistry, Cornell College, Mount Vernon, IA 52314; aault@cornell college.edu.
Vol. 83 No. 11 November 2006
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Journal of Chemical Education
1587
