Chemical Education Today
Letters Solubility and Solubility Products
The authors reply:
Two articles in the September 1998 issue of the Journal of Chemical Education discuss aspects of solubility product constants: using the relevant equilibrium reaction (1), and discrepancies between textbook tabulations of the constants and calculations based on the solubility of a sparingly soluble salt (2). Rather than just using textbook and handbook values, it is revealing to return to the literature for characteristics of each system. Both articles mention the case of CaSO4, the solubility of which is sufficient to make practical its determination in school laboratories. The reason why the solubility product constant calculated from the observed solubility is 10 times greater than that listed in tables has received analysis and explanation in this Journal (3). To the total observed solubility of 18 mM, the zero ionic strength solubility calculated from the solubility product constant contributes only 26%, the neutral salt or activity coefficient effect makes the largest contribution, 47%, and ion pairing contributes 28%. The analysis presented is generally applicable to other salts. Two contributions in the February 1989 issue of this Journal offer a comparable treatment (4, 5). A discussion of the very limited solubility of Fe(OH)3 (1) fails to distinguish between two different quantities: the solubility of free Fe3+ ion and the total solubility of all soluble forms. The first quantity is much, much less than the latter. Revealing distribution curves and concentrations of the free ion and total soluble forms have been plotted as a function of pH (6 ). At pH 7 the concentration of free Fe3+ is only 10᎑17 M, whereas that for the total of all soluble forms is much greater, 10᎑9 M. From the distribution curves we learn that of the soluble forms at pH 7, 28% occurs as Fe(OH)2+ and 71% as soluble Fe(OH)3. For hydrolysis of cations a classic reference should be consulted (7).
Martin is certainly correct that solubility is a more complicated process than simple equations can express. While scholars may have time to return to the literature for the characteristics of each system suggested by a table of Ksp values, lecturers at the secondary and freshman chemistry levels often do not. What they need is a table of Ksp values that will yield reasonable solubilities. That is why we based our paper on empirically matching listed solubilities with simple textbook theory, rather than trying to explain the discrepancies. Correction: In Table 3 of our paper the reference numbering is wrong. Subtract 3 from each reference number to get the correct source for the data. Roy W. Clark and Judith M. Bonicamp Department of Chemistry Middle Tennessee State University Murfreesboro, TN 37132
Literature Cited 1. Hawkes, S. J. J. Chem. Educ. 1998, 75, 1179–1181. 2. Clark, R. W.; Bonicamp, J. M. J. Chem. Educ. 1998, 75, 1182–1185. 3. Martin, R. B. J. Chem. Educ. 1986, 63, 471–472. 4. Carpenter, J. H. J. Chem. Educ. 1989, 66, 184. 5. Russo, S. O.; Hanania, G. I. H. J. Chem. Educ. 1989, 66, 148–153. 6. Martin, R. B. J. Inorg. Biochem. 1991, 44, 141–147. 7. Baes, C. F. Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. R. Bruce Martin Department of Chemistry University of Virginia Charlottesville, VA 22903
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Journal of Chemical Education • Vol. 77 No. 12 December 2000 • JChemEd.chem.wisc.edu
