edited by: RON
DELORENZO
Middle Geoqia College Cachran, Georgia31014
Coffee, Coins, and Limiting Reagents Dennis McMlnn Gonraga University
chiometry, limiting reagents, theoretical yield.exp~rimentnl yield, and percent yield. In my classes the Ereatest benefit has been in explaining limiting reagent.
Spokane. WA 99258
As simple as i t seems t o practicing chemists and even to students who have mastered the idea, the concept of the limiting reagent continues to baffle many beginning students in general chemistry. Invariably, some of these students get confused when faced with having to count particles by weighing and having to be cognizant of the stoichiometry of a reaction. Over the years I have tried many analogies in an attempt to make the basic idea clear a t the beginning of a discussion of the mole concent and stoichiometm. None has been as successful as the one presented here. The students are told the a~oroximare mass of a sinale .. nickel, dime, and q u m r and are asked to calculate how many cunn of calffee can he ourchased it'thcv have 100 r of each coin a k coffee costs $0.50 per cup. ~ s s u m i n gthat the coins weigh 5.00 g, 2.22 g, and 5.55 g (nickels, dimes, and quarters, respectively),' a simple calculation shows that 20 nickels, 45 dimes, and 18 quartem are available--a total of $10.00. In this case, 20 cups of coffee can be purchased. Thus, the idea that mass can be used to count particles is demonstrated easily. The second part of the analogy comes by stipulating that coffee is to be purchased from a machine requiring the use of 1quarter, 2 dimes, and 1nickel. Although they have the same numher of coins and the coffee price has not changed, the amount of coffee available (the "the theoretical yield") is reduced to 18 cups. The quarters have become the limiting reagent. If the "stoichiometry" is altered so that the machine will accept only 1quarter, 1dime, and 3 nickels, a different theoretical vield is nossible since nickels now have become the limiting reagent. Still a further extension is possible if desired. The concept of experimental yield or percent yield could be incorporated bv determinine what would h a.p.~ e nif some of the CUDS were spilled before being counted. This simple analogy is thus appropriate for discussion of the relationship between mass and number of particles, stoi-
Analogies that Indicate the Size of Atoms and Molecules and the Magnitude of Avogardo's Number M. Dale Alexander Gordon J. Ewing Floyd T. Abbot1 New Mexico State University Las Cruces. NM 88003
Students often have difficulty imagining the size of atoms and mol~culesand the marnitude of Avoaardru's numbor. We have arrived a t three analogies which are helpful in overcoming this difficulty. The first of these pertains to the size of atoms. We define a unit of length for short distances in a manner similar to the light year which is used for long distances. The unit is called the beard-second, the length a heard grows in one second. Of course, since this varies from man to man we need a standard. The standard chosen for the rate of heard growth is one centimeter per month. With this standard the heard-second is 3.9 nm which is roughly ten times the diameter of a carbon atom. The analogy pertaining t o the size of water molecules is as follows. Imagine that you have the power to decrease your size. If you were to decrease your size to such an extent that an ant would appear to he one mile tall (five times taller then the Empire State Building) when standing on all six legs then a water molecule would appear to be the size of a grain of salt. Finallv. .. consider an analoev . .. useful in illustratina the magnitude of Avogardro's numher. The volume of the Pacilic O c r m is 7 X 10" krnhhich is 7 X 10" milliliters.' Thus, the number of milliliters of water in the Pacific Ocean is nearly equal to Avogardo's numher, 6 X loz3.
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The usefulnessof the analogy is increased because the masses of the coins are convenient. In fact, students could weigh the coins themselves, with only slight "rounding off needed.
' "Oceans and Seas," Encyclopaedia Britannica. 13,484 (1974).
Volume 61
Number 7
July 1984
591
