The mole: A number of a mass?

---

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.

'

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?