scientist first. She taueht that "good stuff' about molal freezing-point depressidn and alloiropic forms of sulfur and the combined ear law. I loved it and many of the kids I teach today love it, coo. My students do complain, but complaints are not rare in their other classes, either, the teachers tell me. Driver Ed and Health are "too hard" or too "dull". History is "not useful". and foreien laneuaee reauires "too much memorizing", etc. Make no mistake about it. the areument in Kraicik and Yager's article is anti-intellectualism~~ts roots go baik a long wavin education. I t seeks todraw stren&h from - ~rofessional . popular appeal. Its adherents are kids who want an easier program, parents who want the appearance of success on report cards without hassle, a few teachers whose background is weak or who prefer the authority of newspapers to textbooks and research articles. I t is "democratic" in the negative, divisive sense of the word. Teaching objective material does not make a teacher unpopular or turn off the students. When the graduated high school students come back from colleee t o see vou. . . thev- will thank you forgiving them a solid baspof understanding and skills for that touch first vear of colleze chemistrv. Please, give us the "luhber" to build our courses, but let the "sawdust" filter down into education journals.
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S. T. Bond Brldgepon High School Bridgepm, WV 26330
Solubility Products and Solublllty: Plus Fa Change?
To the Editor: Although Masterman (1)in his article "K,? Determination of Calcium Sulfate" has measured the solubtltty of CaS04 in water, he has not determined the solubility product Ksp.The relationship (or lack of it) between solubility and K,, perhaps needsto be emphasized once again, because it appears to be something that is discovered and subsequently forgotten with rnonoionous regularity. In thip Journal alone, the point was fully discussed by 'Meites et al ( Z J , overlooked by Sawver (31. discussed afresh bv Martin (4) and neglected by ~ a G e r n & ( I ) . Briefly, to equate K, to the square of the solubilitv of CaSOa is erosslv in error, because it ignores (1) the appr&iable ionic ilrengfh of the solution (because hoth ions are divalent and the salt is not highly insoluble), which leads to activity coefficients very different from unity, and (2) the fact that an appreciable proportion of the compound is present in solution as the undissociated molecule or ionpair CaS04(aq), i.e., it is not a strong electrolyte. If these factors are taken into account then the true thermodynamic , = ac.2+aso,2- is about 2.5 X solubility product KO at
25 OC (2, 4, 5, 6); this is almost 10 times smaller than the square of the solubility. Meites et al. (2) make the pertinent points that (1)the solubility of CaS04is analogous to that of a sparingly soluble weak acid such as 2,4,6-trichlorophenol, (2) it would be better t o confine the soluhility-product principle to univalent salts such as silver bromide or thallous iodide where there is negligible ion-pairing, and (3) teachers should seriously consider whether the solubility-product principle is in fact closely enough related to solubility to justify its presentation in that context. The above comments may seem to be rather pedantic, but their neglect is illustrated in an illuminating way by the value of the "Solubility Product" for CaSO4 quoted in the CRC Handbook of Chemistry and Physics (7) (hereafter called the CRCHandbook). I noticed that the value of K,, = 1.9 X 10-4quotedfrom itby Masterman (1) was not the same as that in mv own c o ~ vOn . examinine the various editions of the CRC ~ & d b o o k ~ v a i l a b lin e our raboratories, I obtained the results shown in the table. Over a Deriod of 40 vears the value given in the CRC Hondbook has'changed at (east four times (~ossiblvmore. I haven't looked at all editions) with a range o>almo& an order of magnitude. We can now speculate on the reasons for these changes. There is no information in the CRC Handbook about sources, except that the three latest editions have used AGP values t o calculate K, (K,, = exp(-ArGo/RT) where ArGo/ R T = AGfo(CaZ+,aq)+ AGfo(S042-,aq) - AGP(CaS04, c)). The ~reviousvalue of 2.45 X 10-5 nrobablv comes from ref 5 whic'h was published in 1970; t6e CRC- andb book value changed somewhere between the 1969-70 and the 1972-73 editions. The earlier value of 1.95 X was almost certainIv the sauare of the solubilitv, whose value auoted in the .'~hysical Constants of inorganic ~ o m ~ o u n d stable ; ' in the CRC Hondbook has remained constant over the whole ~ e r i od. There are several morals to this tale. Always treat tabulated data in textbooks and compilations with skepticism, especially if the original source is not given; there is no guarantee that the compilers have always appreciated the approximations they have made. Quote the edition as well as the book from which vou obtain data. And do not assume that the data quoted in the latest edition are necessarily more reliable than those from previous editions; in this case the value of 2.45 X 105isclosest to the"trueWthermodynamic value of ref 5, which was ohtained hv careful ex~erimentson CaSOn and is, I suspect, more reliible than thk values in the most recent editions that were calculated from general thermodynamic data obtained from a variety of sources. Llterature Cited 1. Masterman.D. J. Chem.Edue. 1987,64,408-409. 2. Meitea, L.: P d e , J. S. F.;Thomas.H. C. J. Chem. Edue. 1966.43.667472, 3. Sawyer, A. K.J. C h m Educ. 1983.60,416. 4. Martin, R. B. J. Chom. Edue. 1986,63,471-472. 5. Gardner, A. W:. Glueakauf, E. lhns. Forodoy Sm. 1970.66,1061-1087. 6. Guenther, W. B. Ch~micolEguilibtium;Plenum: New York, 1975:pp 172-176 7. CRCHandbook of ChemisfryondPhysics: CRC P r e s B~ocsRaton, FL. John H. Carpenter Newcastle University Newcastle upon Tyne. NEl 7RU. UK
Value of Solublllty Product 01 CaSO. In the CRC Handbook. Edition 301h (i947) 39th, 41st, 44m. 49th, 50th. 53rd. 55-6mh, 62nd
65th. 66M 67th (1986-1987) NO table 01 801UbillN pmducb in 46m
184
Temp.
Kw
6.1 X 1.95 X lo-'
2.45 X 3.73 X 7.1 X
i0W5 iOP
and 47m edBona.
Journal of Chemical Education
10 OC 10 OC 25 OC 25 ' C 25 ' C
review ankle "Ion Association. Solubllliies, and Potemials in Aqueous Solution" by Russo and Hanania. which begins on page 148 of this Issue, also discusses the confusion about CaSO. and places It in the larger context of how aqueous soiutlon concepts should be taught in general chemlstv. Edlor's NOte: A
RBduCtion
