Stolchlometrlc Calculatlons Uslng Moles To the Editor: Many articles have appeared in this Journal recently concerning stoichiometric calculations as taught in introductory chemistry courses (see for example, 1-5). Some espouse specific efficient methods for performing these calculations and others discuss the methods vis-a-vis the ability of students to comprehend them. With exceptions, it appears that the primary aim is to teach students to solve stoichiometric ~roblems.I t seems to me, however, that the most important goal of presenting stoichi&netry iiearly chemistry ciasses is to teach something about how reactions occur on the atomic and molecular level and not merely t o teach prohlem solving. This is particularly true in view of the somewhat frightening (to me) prediction by Brooks (6) that much of our emphasis on chemical calculations will become a thing of the past. Stoichiometry can he explained descriptively, hut it will be better understood by students able to perform quantitative calculations. This is true. however. onlv if the calculations are performed by a method that m&es the student thinkin terms of the fundamental particles reacting. Most of the methods touted do not do this. Ratio methods, reaction equivalent methods, factor-label methods, and others may he efficient and easy to learn and apply, hut they do not get the student thinking in terms of reacting atoms, ions, and molecules. Carrying this view to its extreme indicates that the best method for solvine stoichiometric oroblems is to deal with the actual nuinheis and masses of'individual atoms, molecules. andlor ions. The followine - ~.r o b l e mwould then be solved as shown:
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How many grams of Hz0 will he produced if 10.0 g of Oz reacted with excess Hz according to 2Hz Oz 2Hz0?
+
(1) No. of Oz molecules reacting = mass Ozlmass per O2molecule (2) No. of Hz0 molecules formed = 2 X No. of O2 molecules
reacting (3) mass H20formed = No. of H20molecules formed X mass per
H20molecule
This extreme approach, although forcing the student to think in terms of elementary particles, would he a horror due to the very large and very small numbers involved. But that is whv the mole was invented! Althoueh its use in eeneral chemistry is still being debated (7,8), f d t y (9) shows, and I aeree. that the mole is no more difficult a conceDt than the d&e& Using moles, the steps in solving the above problem become: No. of moles 0 2 reacting = mass 02,mass per mole 0 2 (2) No. of moles H20 iormed = 2 X No. of moles OLreacting 131 mass H?Oformed = No. of moles H20formedX mass H1O per (1)
mole Hz0
factor labels, provides a general, if not always the most efficient tool; hut most important it teaches chemistry. To my disappointment, of the 10 new general chemistry texts we are evaluating this spring, nine use a conversion factor or factor-lahel approach to stoichiometry. A recent "Provocative Opinion" (10) concerning the factor label approach asked, . . is it all that great?" I vote no!
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1. 8teiner.R. P.; Cain, L. J Chsm. Edue. 1986.63,1048,1949.
2. Dierka. W.:Weninger,J: Harron, J. 0. J.C h e m . E d u 1985.62.839,1021. 3. Laionby, J. N.; Morris. J. E.: Waddington, D. J. il Chem. Educ. 1985.62.61.
4. Beiger, S.A. J , Chem. Edue. 1985.62, 396. 5. Cammn. D. L. - J. Chem. Edue. 1984.62610. 6. Brooks, D. W. J. Chem. Edue. 1987,64,53. 7. Gorin, G.J.Chem.Edue. 1987.64.192. S. Bent, H. A. J. Chem. Educ. 1987,64,192. 9. h l w , W.L.J.Chom. Educ. 1985.62.61. 10. Navidi. M.H.; Baker, A. D. J. Chem. Educ. 1981.61.522. ~
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Stewart Karp C. W. Post College Long Island University Greenvale. NY 11568
A 138-Year-Old Error To the Editor:
Ronald I. Perkins' otherwise excellent James Bryant Conant Award address (''Some Things have Changed Little in the Last 138 ~ears",[1986,63, 7811) unfortun&ely propagates a 138-year-old error in quoting from John Scoffern's Chemistr) N O Mjster, or a ~ectuier'r.Bequest (2nd ed.; Arthur Hall: London, 1948; p 172) that "Monsieur Ampere [sicl, the discoverer of chloride of nitrogen, lost an eye and a hand in prosecuting his experiment3 on it;. . .."Pierre Louis Dulone 11785-1838) of Dulone and Petit's law fame. not ~ n d r 6 - ~ a r ~i e m ~ i (1775-1836), r e is universally acknowledeed as the discoverer of nitrogen trichloride. NCL. aithough Louis-Nicolas Vauquelin 71763-1829) obtained the suhstance as early as 1803 without recognizing it as such. These facts were reported to Humphry Davy by AmpBre, which may have given rise to the mistaken impression that Ampere was the discoverer (Davy, H. Phil. Trans. 1813,103, 1, 242; AmpBre, A. M. Schweigger's J. Chem. und Phys. 1813, 8, 323). Dulong unexpectedly discovered "une nouuelle substance detonnante" in October 1811 during an investigation of the action of chlorine on ammonia or ammonium salts of strong acids, 3CI, + NH,CL
Reasonine in terms o ~ m o l e (multioles s of individual Darticles) is thesame process as reaioning'in terms of indivihual ~articles.The understanding of chemical reactions is enhanced-important, and the problem is solved-less important. An additional advantage to thinking in terms of moles is that this approach can he applied to all stoichiometric prohlems, including those involving solutions. The methods mentioned ahove are not all generally applicable without some tortured forcing. Using moles for stoichiometric calculations as shown above, not simply for making conversion factors or
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-
NCI,
+ 4HC1
and in October 1812 he lost an eye and two fingers in an explosion of his new compound (Dulong, P. L. Schweigger's J. Chem. und Phvs. 1811. 5. 122: 1813. 8. 302. Beilaee " 2). . According to J. W. el lo; (A ~ o k p r e h e & i u e' ~ r e a t i s eon Inoreanic and Theoretical Chemistrv: Lonemans. Green: l onion, 1910; Vol. 8, p 598),Dulong 10stthre;fin~er.s and an eye. George 6. Kauffman California State University. Fresno Fresno. CA 93740
Volume 65 Number 12 December 1988
1115
