Chemical Education Today
Letters What a Tangled Web If Michael Laing has his way: “Bring Back Equivalent Weight—If You Want the Kids To ‘Think’!” (J. Chem. Educ. 1996, 73, 1007), introductory chemistry students will gain a much better understanding of what atomic and molecular weights are all about than they do now from half-baked invocations of mass spectrometry. But he’s wrong in one detail. “Oh, what a tangled web we weave…” is from Sir Walter Scott (Marmion), not Shakespeare. A. C. Hall 6231 Tremont St. Dallas, TX 75214 The author replies: Of course your correspondent is correct! The quotation is from Sir Walter Scott and not William Shakespeare. I stand corrected and embarrassed! M. Laing Department of Chemistry and Applied Chemistry University of Natal Durban South Africa
Dissociation Constant for a Monoprotic Acid The article “Determination of the Dissociation Constant for a Monoprotic Acid by Simple pH Measurements” (J. Chem. Educ. 1996, 74, 792) raises questions that teachers may exploit usefully in the classroom. 1. Because of the wide range of pH values in the experiments, one presumes that the pH meter was standardized using NBS buffers or the equivalent. What, then is the meaning of the initial pH (before the acid is added)? If it is taken to yield an approximation to the activity of hydrogen ion, then why do the authors ignore the ionic strength (and activity coefficient) in deducing their values of Bo? 2. The authors conclude that “it is only necessary to know the approximate molecular weight [of the acid] as well as the exact (emphasis added) concentration of the alkaline solution.” and “It is not necessary…to standardize the solutions.” Are they not using the pH meter as a (poor) way to standardize their initial NaOH? Why rely on a meter (an error of 0.05 pH unit corresponds to an error of about 12% in concentration) when it would be so easy to make up the solutions very accurately with standard dilute NaOH and NaCl? 3. What is the value in making a fairly complicated plot to process the data? Since each data point is merely a simple monoprotic buffer solution, rapid deduction of K-values with a calculator is easy. In other words, their equation K1 = x/(Ao-x)/B is all one needs for each point, and is not improved by conversion to a plottable linear form. The answer is that the plot method uses the ratio of
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intercept to slope to find K, and therefore has the advantage that the value of A o actually cancels, thereby justifying their assertion that the molecular weight need not be accurately known. However, isn’t it true that for any acid for which this approach is suitable it is easy to determine the molecular weight by titration, and that this is surely instructive for the students? 4. Further, doesn’t such a plot, using reciprocals, weight the points unequally? With individual point calculations each point is weighted equally in deducing the average pK. Note in the figure that the highest point seems “out of line”. How would the intercept of this plot change when that one point is omitted? (One gets 9.42 for pKa instead of 9.36.) With individual calculations isn’t it easier to note a deviating point or two? 5. In short, doesn’t it seem better to: use standardized NaOH; correct for ionic strength; use an accurate value for the molecular weight; and calculate each point individually? Such an approach is simple, fast, and more reliable, and doesn’t conceal the deviation that any one point may have. Richard Ramette Department of Chemistry Carleton College Northfield, MN 55057 The authors reply: 1. The ionic strength is maintained constant in all experiments, since we always work with an excess of NaCl. Thus, all the pH measures suffer similar effects. 2. Because the pKa values we obtain are very similar to those obtained from the literature, and only for this reason, we wrote that “It is not necessary, therefore, to standardize the solutions”. Of course, working with titrated NaOH will improve this agreement. 3. We think that the plot of 1/x versus 1/B helps to visualize the consistency of data, and usually a quadratic minimization with several points linearly distributed (r > .99) gets better results than averaging. The determination of the neutralization equivalent by titration is a very instructive and classical experimental procedure, but our objective with this paper entitled “Determination of the Dissociation Constant for a Monoprotic Acid by Simple pH Measurements” was only to show a very elemental method for the pKa determination. 4. Unequal weighting of the points, due to the use of reciprocals, is an interesting question that would deserve careful consideration. As for the deviating points, they can be easily detected as “out of line” in the proposed plot. 5. We think that the results obtained with the acids tested in this work guarantee sufficiently the reliability of the proposed method. However, if we want greater accuracy in the pKa determination, it would be advisable to use standardized solutions and an accurate value for the molecular weight of acid.
Journal of Chemical Education • Vol. 74 No. 8 August 1997
R. García-Doménech, J. V. de Julián-Ortiz, G.M. Antón-Fos, and J. Gálvez-Alvares Universitat de València 46100 Burjassot Valencia, Spain
Chemical Education Today
Letters Thus scientific progress has disproved another of the “factual truths” that many of us learned in school. Time marches on.
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About Letters to the Editor Letters to the Editor may be submitted to the editorial office by regular mail (JCE, University of Wisconsin–Madison, Department of Chemistry, 209 N. Brooks, Madison, WI 53715-1116), by fax (608/262-7145), or by email (jce@ chem.wisc.edu). Be sure to include your complete address, your daytime phone number, and your signature. Your letter should be brief (400 words or less) and to the point; it may be edited for style, consistency, clarity, or for space considerations.
The Dimensions of Logarithmic Quantities I am surprised and puzzled by the absence of published comment on the proposal by Molyneux (1) to introduce logarithms of dimensioned quantities. It is not clear whether Journal readers (a) find no objection to this proposal, or (b) find the proposal so outlandish as to be unworthy of comment. My own view is the latter—except that I believe that a strong objection to this proposal should be on record in the Journal, and that readers should be reminded that the proper way to handle the problem that Molyneux purports to solve was published in this Journal almost 40 years ago (2, 3). Literature Cited 1. Molyneux, P. J. Chem. Educ. 1991, 68, 467–469. 2. Boggs, J. E. J. Chem. Educ. 1958, 35, 30–31. 3. Copley, G. N. J. Chem. Educ. 1958, 35, 366–367.
George B. Kauffman California State University, Fresno Fresno, CA 93740-0070 email:
[email protected] To Weigh It appears that the perfectly satisfactory verb “to weigh” is being displaced by an inappropriate verb “to mass”. Yes, we do know the difference between the nouns “weight” and “mass”, but that is not the issue here. According to the Random House dictionary the verb “to mass” means to gather into or dispose in a mass or masses, or to assemble; whereas “to weigh” means to ascertain the force that gravitation exerts upon (a person or thing) by use of a balance, scale or other mechanical device. The article by Roger Plumsky (J. Chem. Educ. 1996, 73, 451–454) contains some interesting experiments, but we were disturbed by the fact that students are told to mass a test tube, crystals etc. Surely what chemistry students do when they go to the balance is “to weigh” a sample or object! Marion W. Smith and Charles E. Russell Department of Chemistry Muhlenberg College Allentown, PA 18104
Correction
Robert D. Freeman Professor Emeritus, Oklahoma State University Stillwater, OK 74078-3071
In the article “Convergent Synthesis of Betaine-30, a Solvatochromic Dye: An Advanced Undergraduate Project and Demonstration” by Bruce R. Osterby and Ronald D. McKelvey (J. Chem. Educ. 1996, 73, 260–261), we regret that the 9 references in the paper were not printed. They are printed below and the complete article is available on JCE Online (http://jchemed.chem.wisc.edu).
Chemical Magic: An Update Shortly after the publication of the late Ray Seymour’s and my “Products of Chemistry” article, “Chemical Magic: Polymers from a Nonexistent Monomer” (J. Chem. Educ. 1994, 71, 538), Thomas T. Tidwell of the Department of Chemistry, University of Toronto, kindly called my attention to a article reporting the preparation and polymerization of vinyl alcohol, the “nonexistent monomer” of our title. Anna K. Cederstav and Bruce M. Novak of the University of California, Berkeley have succeeded in polymerizing almost quantitatively vinyl alcohol (H2 C=CHOH), the thermodynamically unstable enolic tautomer of acetaldehyde (H3CCHO), generated in situ by the hydrolysis of a precursor (ketene methyl vinyl acetal) and siphoned off faster than it can tautomerize to acetaldehyde (J. Am. Chem. Soc. 1994, 116, 4073). The first report of hydrolysis conditions that yield metastable vinyl alcohol solutions with a significant lifetime at room temperature makes this previously “nonexistant monomer” a synthetically useful substrate. The two Berkeley chemists are investigating alternative catalyst systems that may allow them to carry out the homopolymerization of vinyl alcohol and other thermodynamically unstable enols.
Literature Cited 1. Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, 1988; p 288. 2. Streitwieser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; Essay 4, p 621d. 3. Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs Ann. Chem. 1963, 661, 1–37. 4. Reichardt, C. Angew. Chem. Int. Ed. 1965, 4, 29– 40. 5. Reichardt, C. Angew. Chem. Int. Ed. 1979, 18, 98– 110. 6. Durst, H. D.; Gokel, G. W. Experimental Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1987. 7. Chadwick, T. C. Anal. Chem. 1974, 64, 1326. 8. Kessler, M. A.; Wolfbeis, O. S. Synthesis 1988, 8, 635–636. 9. Johnson, B. P.; Gabrielsen, B.; Matulenko, M.; Dorsey, H. G. Anal. Lett. 1986, 19, 939–962.
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