To the Editor: We a n e e with Baldwin and Burchill's analysis of thermodynamL equilihrium constants. However, we were discussing Law of Mass Action K. values, which is the approach usually presented in lower division undergraduate chemistry courses. The K. values based on thermodynamic equilibrium constants using the standard conventions for electrolyte solutions (i.e., based on Raoult's law for the solvent and on Henry's law for the solute) do make water and hydronium ion special cases because of the conventions employed. We agree that "Perhaps the apparent confusion.. .would be reduced if equilibrium ratios were always written first in terms of activities. . .",hut we feel that it is improbable that this will happen in introductory textbooks and courses in the near future. I t is likely that students will continue to be introduced to the concept of K, values via the Law of Mass Action We believe that it is, therefore, appropri- - ~ - annroach. ~ ate in an instructional context to treat water and hydronium ions in a manner that is consistent with the conventions and approximations of the Law of Mass Action as i t is used to develop K, values for other acids. The letter of Baldwin and Burchill amplifies the need to distinguish carefully and identify the approach used in equilibrium calculations. We thank Hyuk Yu for his helpful discussion on the suhject.
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University of Wisconsin. Green Bay Green Bay. WI 54302
Slgnlflcance and Precision To the Editor: The September 1985 issue of this Journal offers a rare double opportunity to point out misconceptions in the treatment of data. The first misconception occurs in the IUPAC Table of Atomic Weights to Four Significant Figures, prepared by N. N. Greenwood and H. S. Peiser (p 744). The limitation to four significant figures implies that the precision of the atomic weights has been kept approximately constant. While that is true in an absolute sense-the authors point out that the values in the table are reliable to f 1 or better in the fourth significant figure except for five elements for which larger indicated uncertainties a .~ ~-l v - i tis ~~-~ completely false in relative terms. Specifically, the relative error in atomic weieht varies bv a factor of 10: from a low of 0.01% for beryllium (9.012) and molybdenum (95.94) t o a high of 0.1% for hvdrogen . - (1.088). boron (10.81), and ruthenium (101.1). Furthermore, t'or use in stoirhimnetric calculations, atomic weights gi\.en to the same number ot' decimal points are generally more useful; otherwise, siynificant figures are lost when formula weights are calculated, thereby limiting the precision of many types of nnalytiral determinations. For example, with the data in the table rhe formula weieht of silver cbluride is 113.2. 1 therefore recommend that the table he revised so that all the atomic weights
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A. V. Hartkopf Mobil Chemical Company Edison. NJ 08818
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Ronald Starkey Jack Norman Mark Hlntze'
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over 100 in the table are expressed to 0.01 units. This would double the range in relative precision hut would make the table more realistically useful. The second statistical misjudgment occurs in the article "Modern Inorganic Chemistry as Found in the Contents of SevenPost-1980 Textbooks", by P. S. Poskozim (p 747). The comparative coverage of various topics in inorganic chemistry by the textbooks is determined by averaging the number and percent of the pages in the text that discuss a given topic. In some cases the range of coverage is highly skewed. The most extreme example of this problem occurs with descriptive chemistry, which is represented in the seven textbooks by the following page counts and percentages: 100 (l5%), 28 (5%), 139 (IS%), 301 (29%), 84 (lo%), 956 (70%), and 220 (36%).The averages are 261 and 26%. I t is clear that hoth of these values are too high to be eood measures of the -~~~~~ central tendency of the data; &e effect-of the 956 pages and 70% is too great. In cases such as this, the median is a better measure of central tendency than the average; the corresoondine medians are 139 and 18%;these should have been used for:he rest of the discussion.
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Journal of Chemical Education
To the Editor: A. V. Hartkopf alleges that a "misconception occurs in the IUPAC Table of Atomic Weights to Four Significant Figures.. ." in that "The limitation t o four significant figures implies that the precision of the atomic weights has been kept approximately constant." By currently assessed best knowledge of the atomic weights-as given in the full IUPAC Tahle of the Standard Atomic Weights published biennially in Pure and Applied Chemistry-their uncertainties vary (from the least to a most precisely tabulated atomic weight) by a factor of 10,000. The Table to Four Significant Figures thus happens to fulfill Hartkopfs implication as stated by him. The IUPAC Commission on Atomic Weights and Isotopic Ahundauces was requested t o prepare an Atomic Weights Table for students' use by eliminating those significant figures in the full IUPAC Table that for elementary work were irrelevant and confusing. The authors designated for this task by the IUPAC Commission willingly accepted these terms of reference, but in the accompanying text guided users t o the full IUPAC Tahle for any available additional accuracy, whenever the abbreviated value was inadequate for the intended application. One could offer an atomic weights table to a more constant precision, but wirh n variable nimber of significant figures. It would have the disadvantage of pn,hahly needing more frequent revisions in the light of significant new experimental data than the existing IL'PAC Table to Fuur Significant Figures. A table of atomic weights to a constant number of decimals, as suggested by ~ a r t i o ~would f , have even greater problems, especially for organic molecules. Hartkopfs atomic weight for hydrogen needs to be corrected from 1.088 to 1.008 (to four figures). Far less forgivable is my own error in the IUPAC Table t o Four Significant Figures, where the atomic weight for niobium should read 92.91 not 92.21! H. S. Pelser
