Chemical Education Today
Oxygen vs Dioxygen: Diatomic/Monatomic Usage Are you as tired as I am of having freshman chemistry students who divide by 32 when determining the empirical formula of a compound containing oxygen or by 35.5 when finding the number of moles of gaseous elemental chlorine in a stoichiometry problem? How about students who persistently write weak acids as ions when writing gross and net ionic equations? I am using two techniques that have all but eliminated these errors in my classes and hope, by this letter, to influence you to do the same. Here is what I do, thanks to the example of Bill Plachy in the Department of Chemistry at San Francisco State University. I consistently, in writing and speaking, refer to hydrogen gas as “dihydrogen”, chlorine gas as “dichlorine”, nitrogen gas as “dinitrogen”, etc. I use “hydrogen” only when referring to the monatomic species. In the case of weak acids, the subscript (aq) is misinterpreted by many students to mean that the substance ionizes completely, as it does when used for salts. Of course (aq) means only that the substance is soluble in water. Strong acids and bases can be labeled (aq) without confusion because they too ionize completely. Why not label weak acids, instead of (aq), as (wa), weak acid? Weak bases could be labeled (wb). If you agree with these ideas, please feel free to adopt them in your class presentations, written handouts, and the next edition of the text you have written. If enough people start to adopt this system it will become commonly used, and we will have blotted out these two errors once and for all. Jared B. Sharon Department of Chemistry Canada College Redwood City, California 94061
Ionization or Dissociation? I applaud the article by Emeric Schultz in the July, 1997 issue of the Journal titled “Ionization or Dissociation?” Finally someone has raised the issue of the confusion surrounding these two terms in elementary texts and begun the dialog to correct it. The article is logical and consistent as far as it goes, and I concur with its final recommendation. However, a set of definitions for these two terms that could be used by educators and authors working at the introductory level is needed. As a starting point, and after consultation and recommendations from Schultz, I propose the following definitions:
In general Ionization: a chemical change involving the transformation of an uncharged species into an ion (or ions) or other charged entities. Dissociation: a change involving the separation of a chemical entity into its component parts without creation of new ions or other charged entities.
In aqueous solution Ionization: a chemical change involving the transformation of water into H3O+ or OH– or both. Dissociation: a change involving the separation of a chemical entity into its component parts in water without creation of H3O+ or OH–. Hopefully others will add to and enhance these definitions so that we can communicate these concepts as clearly as possible. David L. Adams Babson College Babson Park, MA 02157-0310
Cheating Probabilities The note by Rizzuto and Walters on “Cheating Probabilities on Multiple Choice Tests” (J. Chem. Educ. 1997, 74, 1185) implies that a significant measure of the probability that two examination papers were not created independently can be obtained using the binomial probability distribution. They do point out that the probability of correlated answers is most probably higher than predicted by the formula, and suggest a value of .001 may be appropriate for considering the possibility of cheating. It can easily be shown that this limit may be completely inappropriate. For example, consider an examination with 50 questions. If two students each get an 80% on the examination, the minimum number of questions that will have the same answer is 30, (each misses 10 questions but a different 10 questions). The probability of this occurring by chance, using Table 1 is 5.8 ⫻ 10–10. This would then suggest that it is very unlikely that two students would have exam grades around 80% without cheating! In summary, the reason that “it is not well known that the probability of a student having the same answers as another student can be easily calculated using the binomial distribution” is that it is not true. Charles D. Jonah Argonne National Laboratory Chemistry Division Argonne, IL 60439
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JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education
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