edited by: RON DELORENZO Middle Georgia College Cochran, Georgia 31014
Gram Formula Weights and Fruit Salad The Pennsylvania State University Wilkes-Barre Campus Lehman, PA 18627
In exnlainine the concents of mam formula weieht and the mole to my genwnl chemistry classes, I first dcscrihe thescale of relotive atomic wcirhrs and formula weiehts and then define " the gram formula weight (the formula weight expressed in erams). The reason for definine this term lies in the fact that we oftkn wish to know how many atoms or molecules are in a sam~le.but thev are too small and too numerous to count. We have no "moleche meter" in the laboratory, only the balance for weighing out samples. Suppose, for example, we want to weigh equal numbers of Hz molecules and C atoms. We know that their relative weights are 2 and 12, respectively. It then follows that we would have equal numbers of formula units if we used weights in this ratio, e.g., 2 tons Hp to 12 tons C, 2 lb to 12 lb, or 2 g to 12 g. Frequently, students are unable to "see" this conclusion immediately and hence need some convincing. To that end, I have found the following analogy to be highly successful. Let's suppose you want tomake fruit salad. The recipe says to mix equal numbers of grapes and cherries. (It's a very simple fruit salad). So you go to the grocerystore and request, "I'd like equal numbers of grapes and cherries, please." The grocer replies, "Sorry, we sell them by the pound. How many pounds of each do you want?" Well, you ponder, if I bought the same number of pounds of each, I would have equal numbers of grapes and cherries only if a grape and a cherry have identically the same weight. That's probably not the case. I need to know their relative weights. Meanwhile, the grocer has walked away, enahling you to borrow a stray grape and a cherry from the fruit stand. You quickly weigh each on the nearhy scales and find that one cherry weighs '14 OZ. and one grape weighs I'8 oz. Now you know that a cherry is twice as heavy as a grape (their relative weights are 2 to 1). You therefore decide to buy 2 lb. of cherries and 1lb. of grapes. T o convince everyone that you have indeed purchased as many grapes as cherries, we can readily calculate the number of each from the data at hand. You have:
16 oz.
1cherry
-(I lb. ) ( oz. 16oz. 1grape (I lb. grapes) - ( 1 lb. ( Va )
(2 lb. cherries)
Wayne L. Felty
=
oz
128 cherries
=
grapes
Back to molecules and atoms. One gram formula weight of Hp (2 g of Hp) and one gram formula weight of C (12 g of C) contain the same number of formula units. In fact, one gram formula weight of anything contains the same number of formula units. This number happens to he 6.02 X loz3and is called Avogadro's Number, and this many of whatever items you are counting is called a mole.
Limiting and Excess Reagents, Theoretical Yield Ernest F. Silversmith Morgan State University Cold Spring Lane and Hillen Rd.. Baltimore, MD 21239
Students often find i t puzzling that the reagent present in lesser molar amount is not always the limiting reagent and that the amount of limiting reagent determines the theoretical yield. We ask them to think about the assembling of bicycles. The "equation," frame 2 wheels bicycle, is analogous to a chemical reaction of the form A 2B P, where P represents product. If we start with 15 frames and 26 wheels, the wheels are clearly the "limiting reagent" (even though there are more wheels than frames) and the "theoretical yield" is 13 bicycles. For reactions of the form A B P and A 3 8 P, we switch to unicycles and tricycles, respectively. The class may find it amusing and educational to find analogies for other reaction types. For A + 2B 2P, one possibility is this: 1popsicle (the kind that can be separated into two parts and thereby feed two youngsters) 2 hungry children 2 happy children. Thus, 8 popsicles and 14 hungry children would yield 14 happy children, with an "excess" of 1popsicle.
+
-
--
+
+
-
+
-
-
+
This feature presents a cailectian of descriptive applications and analogies designed to help students understand some of the difficult conceots . lreouentiv ~, . encountered in chernistrv. Contributions that will proa x e a greater applecialoon and rnowlcdge of poiit ca relG 0.s. econom C, historfca ana sc ent t c aspects 01 8le arc enCOLrngM
.
Volume 62 Number 1 January 1985
61
