edited by
J. DUDLEY HERRON Pvrdue University West Lafoyene, Indiana 47907
The Mole Concept Perhaps no concept in chemistry is more basic or more difficult for beginning students than that of a mole. In this issue of High School Forum we present ideas which have helped two teachers place mole in the minds of their students. We hope that you will try them in your class and, if you have a better idea, send i t along so that we can share i t with those who are still trying to make mole as easy as dozen.
Teaching Moles is Child's Play The following idea is abstracted from a brochure prepared by Donald F. Clausen, University of WisconsinStout. Students are told that they are working for a toy manufacturer. Under design and development is a doll's dress and a toy garage. Mock-ups are presented in the form of a cardboard cutout of the dress with buttons glued on and a wooden garage nailed together with four 4-penny finishing nails and six 3-penny box nails. The doll dress requires 3 large buttons and 2 small buttons per dress. Students are told that others in the firm are responsible for design of the dress and garage. Our responsibility is to buy the proper numher of buttons and nails. Sales tells us that they can sell 100,000 doll dresses and 100,000 toy garages. Since the prohlem is the same for hoth articles, only the example of the doll dress is used here. However, in the classroom the redundancv of workine both exam~lesis heloful. I t is immediately eXdent that d a ~ af millfon buttons will be reouired for the 100.000 doll dresses:. 300.000 . of the l a r-~ e size and 200,000 of the small size. The question is how to order them, receive them, and know that we are getting what we pay for. Sooner or later a student will suggest that there must he a relationship between button weight and the number of buttons. The instructor then offers the sueeestion that the calculations be done in terms of 1000 buttons and suggests the name of "toymole" to refer to 1000 units. Since the large buttons weigh 0.4 n each, 1000 of them weigh 400 g or 0.88 lbs. The factory obviously needs 300 "toymoles" of large buttons which weigh 264.3 lh and cost, at $11.35 per lb, $3,000. Using similar calculations, the cost of the small huttons is calculated, The calculations are done using English units to strengthen the student's understanding of the relationship between the metric and English systems and t o ensure that the example is meaningful to those students who are not yet able to think in terms of metric units alone. Editor's Note: Contributions in High School Forum from high school teachers are especially solicited. Some teachers may feel that they have more questions to ask than answers to give. These are also invited. Send two copies of all contributions ta the column editor.
After the calculations are completed, the instructor points out that the use of similar units is commonplace. For example, we buy shoes by the pair, eggs by the dozen, and paper by the ream. As things get smaller and smaller, the number of them per unit gets larger and larger; shoes are heavy and come in twos, eggs are lighter and come in twelves, paper is still lighter and comes in 500's. Our small buttons and nails come in 1000's. In order to transfer the idea to atoms and molecules, the instructor points out that atoms are uery small and that the unit used to discuss them is uery large-much larger than the 1000's in which the buttons and nails were measured. I t is pointed out that 1000 atoms or molecules would be much too small to see or to weigh. The real mole is then introduced as the number, - 602,00~000,000,000,000,000,000 (or 6.02 X loz3). Comparisons are then made between the weight of a "toymole" (1000) of large buttons (400 g) and the weight of a "toymole" of small buttons (150 g). The discussion then moves to the weight of a mole of hydrogen atoms, carbon atoms, and ultimately to the weight of one mole of various molecules.
The Mole Calculator The following idea was suggested by Howard F. Heup, Fox Valley Lutheran High School, Appleton, Wisconsin. The "Mole Calculator" (see figure) is a learning aid that has been used successfully a t Fox Valley for four years. I t has been used with high school students of average to above average ability at all grade levels. In the early stages of learning the interrelationships between moles, molecular weight, and the molar volume of a
The mole calculator.
Volume 52, Number 11, November 1975 / 725
gas, students are allowed to use the d c u l a t o r to convert from one unit t o another. Students soon put the calculator aside as familiarity increases and they recall the numerical and mathematical relationships. T o use the calculator, a student locates the known unit of measure on the calculator. He then follows the path indicated on the calculator to the required unit of measure, doing the indicated calculations and using the numerical constants as he goes. The student quickly sees that the mole is a t the center of four important conversions frequently encountered in chemistry. Because the calculator makes the conversions routine, students have less fear of chemistry and the instructor has more time to deal with the meaning of the basic relationships that are shown on the calculator.
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lete." Consistent with this change in terminology, perhaps the Mole Calculator described by Heup should list "Mole Weight" or "Molecular Weight in Grams" in place of "Gram Molecular Weight.'' Is mole a number or is i t a mass? Well, neither. It is an amount of substance and since we can describe the amount of suhstance hy indicating its mass or the numher of elementary particles i t contains, we use the term for both. Until students recognize this, the mole will remain a very confusing concept.
The Identity of Chemical Substances: A First Laboratory Experiment for Elementary Chemistry Students
The Mole: A Number or a Mass? Perhaps one reason that students become confused over the mole concept is that we teachers seem t o he confused ourselves. We talk about the mole as though i t is a number and in the next breath refer t o the mass of a substance in terms of moles. Is mole a number or is it a mass? The definition for mole recommended hv ISO, IUPAP. IUPAC, and CIPM is1 The mole is the amount uf suhstanee of a system which contains as many elementary entities as there are carhon atoms in 0.012 kilopamu of carbun-12.The elementary entity must he specified and may be an atom, s molecule, an ion, an electron, etc., or a specified group of such particles. Since amount of a suhstance is normallv descrihed in terms of its mass, the mole clearly refers to mass. However, as the definition clearlv imnlies. .i t is the mass of the Avoeadro number of particies-any particles. The fact that the mole can refer to the mass of 6.02 X loz3particles of any kind does seem to place emphasis on the number implied by the term and one is tempted to simplify matters by defining mole as the name for a number just as dozen is the name for a number. This is the approach that is suggested by Clausen's analogy with toy manufacturing descrihed above, and it is an approach used by some authors of elementary texts as well as the editor of this column. Such an approach may not he strictly correct, hut i t does seem to make the idea easier for some students t o grasp. Whether the mole is introduced as a name for the number, 6.02 X 1023, or a name of the mass of that number of particles, the emphasis is still placed on the number of particles involved. This, it seems, is a slightly different emphasis than that found in the older definition of the mole as "the molecular weight or formula weight of a compound expressed in mass units." Here, the emphasis is clearly on mass. This older definition limits the application of the term, mole, to compounds and requires such terms as mam-atom when referring- to the Avogadro numher of atoms, or gram-ion when referring to the Avogadro number of ions. With the revised definition of the mole, "units such as the 'gram-molecule,' 'gram-equivalent', 'equivalent', 'gram-ion', 'gram-atom', and 'gram-formula' are all ohso-
'Kirk and Othmer, "Encyclopedia of Chemical Technology, Supplemental Volume," 2nd Ed., John Wiley and Sons, 1971, p. 991.
726 1 Journal of Chemical Education
J a c k E. Fernandez Uniuersity of South Florida Tampa, Florida, 33620 One of the first chemical problems that early chemists faced was establishing the identity of a pure substance. The question might have been put: "How can I determine if this yellow metal is gold?" The modern chemist still asks the question every time he prepares a new compound. The experiment below poses this question and provides an enjoyable, simple, and instructive experience through which to introduce the heginning high school or college student to chemistry during his first laboratory period. The Experiment
We aive each student two test tubes each containing a pure solid suhskaoce, and we ask him to determine whetier the suhstances are the same or different. We aive him no instructions except in the use of water, dilut;! acids and bases, Bunsen hurners, magnifying glasses, and other available common substances and pieces of apparatus that we wish to place a t his disposal. After a brief period of confusion most students realize that nearly any test that shows a difference in behavior establishes that the two suhstances are different, but that failure to demonstrate a difference does not prove that the substances are identical. In the ordinaw 3-hr laboratorv neriod students are able to examine seve;al pairs of suh&&ces. We have found it best to use quite different substances in the first pair. Examples of simple first pairs are sucrose-sodium chloride, sucrose-benzoic acid, and sodium chloride-sodium carbonate. The second pair might then he more similar in their behavior: examnles are sodium chloride-notassium chloride, or perhapseven a pair of identical ~ a & ~ l e Such s . pairs are ideal for demonstrating that two samples are identical only if all of their properties are identical including those that the student is not aware of. At the end of the experimental period we ask each student to tabulate and report all tests and results for each sample of each pair, and to answer questions such as the following: (1) Is it easier to prove that two samples are the same, or to prove that they are different? Explain. (2) Why are color and odor sometimes deceptive in establishing the identity of two substances? (3)What role does purity play in establishing the identity of two samples? (4) If you had to perform these experiments again, how would you change your approach?
