1. H. Bowman Southwest Minnesota State Colleae Marshall, 56258 C. M. Shull Son Diego State University San Diego, California 921 15
II I
Teachers of elementary chemistry are continually searching for experiments that possess the intrigue of something unknown; that have justification for conducting them; that provide opportunities for students to exercise their inductive powers; and that review, apply, and introduce fundamental skills and principles. Historically, a number of supposedly simple chemical reactions have been used to illustrate the basic laws of stoichiometry. These reactions have suffered from a degree of intellectual sterility, safety hazards, nonstoichiometric combinations, etc. Some classical examples include the elemental synthesis of copper sulfide and magnesium oxide; the oxidation of zinc with hydrochloric acid; the metathesis reactions of mercuric nitrate and ammonium thiocyanate and of ammonium sulfate and alkali chlorides; and the decomnosition of mercuric oxide.. ~ .o t a s s i u mchlorate. and metallic carbonates. was developed ~h~ experiment described in this as part of the Physical Science Grouplslproject to imDrove the training of future teachers of the physical scifeatures the developmeit of textual ences. This materials conducive to the inquiry approach to learning. "Mysterious Stoichiometry" is the culminating experi-
'Haber Schaim,Group, Uri, "Experimental Chemistry," Pilot Edition, Physical University, Boston, setts, 1971, pp. 2-5. 21ntroductow physical Group, L'Callege prentiee-~all,Englewoad New Jersey, Physical 1969, p. 174. ~ c.College ~ Introductory ~ , qntmductory physical science G Physical Science," Prentice Hall, Englewood Cliffs, New Jersey, 1969, p. 151. 4Introdnctary Physical Science Group, "College Introductory Physical Science," Prentice Hall, Englewood Cliffs, New Jersey, 1969, p. 222. SIntroductorv Phvsical Science Grauo. "Phvsical Science 11." Pilot Edition, ~ducakon~ e v e l o ~ m e ninter, t lg70, p. 13-2.
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/ Journal of Chemical Education
Mysterious Stoichiometry
ment in a series designed to illustrate the fundamental nature of the atom: its quantized energy nature, its electrical nature, and its combining ability. The experiment is preceded by numerous experiences illustrating these properties, i.e., the emission spectra of excited atomsz and the electrolysis of water,3 copper (I) and copper(II) solutions,' and of lead and nickel salt solution^.^ The latter are used to illustrate the law of definite composition, the specific charged nature of ions, the law of multiple proportions, and to set the stage for posing the question: Aside from accepting the label on a reagent bottle how does a chemist really know what formulation he is using? The Experiment: A Compound of Chromium
The composition of chemical species is determined hy one of a number of means: direct preparation from the elements, decomposition of compounds into simpler recognizable substances, inferential reasoning based on the reaction of known substances. The student's task in this experiment is to determine the composition of a compound of chromium produced in an electrolytic , . c . . .
A piece of pure chromium metal of known mass is made the positive electrode. The negative electrode is a preweighed stainless steel screen. The electrodes are immersed in 50 ml of 0.3 M KOH and connected to a power supply. Current is maintained at
-1 .A..for -.-1 .hr...
The potassium hydroxide is neutralized with 1 M nitric acid and the golden-yellow solution titrated with 0.15 M lead nitrate solution. The absence of further precipitation upon the addition of titrant is accepted as the equivalence point. Detection of the endpoint is at best difficult and is improved by thermally inducing coalation. Students are, told the approximate titration volume, 35-42 ml, in advance. The precipitate is then filtered, at 100"C,andweighed. Comments
A student should experience success in the laboratory. All too frequently a beginning student follows a set of directions which lead him to a result he recoanizes as ahsolutely unreasonable. The consequences ar; that he loses
confidence in himself and the laboratory. In order to assure a hieh success ratio and the buildine- of ~.o s i t i v eself - -concepts we routinely summarize class data at the blackboard (see the table). The data is carefully analyzed in a group, 'post-laboratory discussion. Where applicable, nomograph~are made. No longer does the student with data far from the theoretical value feel "be blew it," nor does he leave with misconceptions based on his inaccurate findings. Rather he observes his data as a part of the statistical whole. The preponderance of data points around the theoretical value is convincing evidence and cause for him to ask, "where did I go wrong?"-a question he can now ask without self-condemnation. While the experimental situations described in this paper from an analytical point of view are crude, the degree of accuracy required for meaningful analysis is adequate. The experiment provides a wealth of opportunity for productive group interaction. During the electrolysis a number of questions are posed
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What would happen if the alligator clip holding the chromium contacted the solution? What is the gas emanating from the negative electrode? Can you experimentally identify it? What is happening at the positive electrode? How many moles of chromium dissolved? What is the chareelatom ratio for chromium? " , Is the chromium species positively or negatively charged? W h a t ir the mass ehanee at the neeative electrode? " How big an error is introduced by overtitration of 1ml? Did you note a color change during the neutralization step? ~
~
~
These questions arouse some interesting class speculation.
Continuation of the experiment and subsequent comparison of the moles of lead required to titrate the chromium and the mass of precipitate-recovered lead to the conclusion that there is some missing mass. Skillful handling of the data, observations, and scientific reasoning ultimately results in concluding that oxygen must account for the missing mass, and PhCr01 is the yellow solid formed. A typical postlah discussion would lead to answers to the suggested questions: hydrogen is evolved at the negative electrode; a yellowish colored species emanated from the positive electrode-no gas evolved; no mass was lost a t the neeative electrode . . . The calculations provide a great opportunity t o review and/or introduce a number of basic stoichiometric coucepts. Calculations of the number of moles of Cr dissolved requires the application of the mole concept, the charge to atom ratio, the application of Avogadro's number, and the fundamental charge (a value our students have previously evaluated); the mass of Ph requires the application of volumetric analytical arithmetic. Ultimately the law of definite composition evolves. The arguments can he extended to include the evidence for a multiatomic anionic species and complex ions in -general. One can also draw attention to the observation of color change accompanying the neutralization of the alkaline solution, and use it to initiate a discussion of dimerization and equilibrium.
Acknowledgment The authors wish to acknowledge the assistance of the National Science Foundation Undergraduate Pre-Service Teacher Education Program.
Volume 52, Number 3, March 1975 / 187
