JOURNAL OF CHEMICAL EDUCATION
534
yet the limitation to saturation with HIS is not even mentioned. Neither historically nor in practice is qualitative analysis, for which these prohlems are apparently presented, based on such calculations for its development or application. These "problems" are introduced, it is agreed, for their usefulness in the teaching of theoretical principles, because the understanding of the principles is proved, ultinately, only in their application to real problems. But for a real problem the student then should deride for himself whether a particularly simple final approximation is applicable to the data. This is the test of the understanding of the chemical and mathematical relations involved. The rest is numerical substitution, non-chemical activity. If then we concern ourselves with making up prohlems which will be assumed in advance t o conform to the simplest final approximations, the student need only follow certain simple typical procedures to "solve" them. Such problems become essentially algebraic exercises without chemical significance; in a sense the answer to the problem is already assumed in the very form of the final approximation used. I n doing this we may succeed in making life easier for the student (and for the instructor) while in school. But in thus avoiding the necessity for the student to think for himself, we misrepresent the real nature of chemical problems and fail in our purpose of fundamental training. JOHN E. RICCI
useful. The several specifie critirisms may now be considered in order. Problem 1. We did fail to insert, after "NaHD," the qualifying phrase, "in which HD- is weak or intermediate in streneth .- both as an acid and a base." The meaning of these strength designations is given in reference ( 2 ) but may be reviewed here briefly. Strenglh designation
sf
conjugate acid
Slrength designation Aridity scale
pKa
of
conjugate base Strong Feeble . . . . . . . . . . . . . . . . . . .2 . . . . . . . .12 . . . . . . . . . . . . . . . . . . . . . . . Intermediate Weak
-
P K ~
-
. . . . . . . . . . . . . . . . . . .I .. . . . . . . . . I . . . . . . . . . . . . . . . . . . . Weak
Intermediate
Feeble
Strong
. . . . . . . . . . . . . . . . . . . .12 . ......... 2. . . . . . . . . . . . . . . . . . . . . . .
The troublesome examples given by Professor Ricci were NaHSOa (HSOn- ion is a strong acid hut a feeble base) and NaHS (HS- is an intermediate base but a feeble acid). Insofar as the pH's of their solutions are concerned these substances may be treated adequately by considering the first merely as a monoprotic acid and the second merely as a monoprotic base. The reason for this is that the solvent, water, is both a feeble base and a feeble acid (pKal = 15.7, pKb, = 15.7) of comparable or greater strength than bisulfate ion as a base or hydrosulfide ion as an acid. Problem 2. The same qualification applies as in problem 1. NEW YORKUNIVERSITY Problem 3. No comment. NEW YORK,NEW YORK Problem 7. At the beginning of the section in which this problem occurs the limitation to salts whose anions are weak or intermediate bases is expressly stated (the cations were neelected but should b~ non-acidic). To the Editor: In Professor Ricci's example, zinc sulfide, the anion is a Professor Ricci's above letter concerning the recent paper by myself and K. Geller may be considered to strong base. Problem 9. At the beginning of the section in which consist of two parts: (1) a general criticism of the probthis problem occurs the term, "slightly soluble salt" lems used in qualitative analysis; (2) specific critiappears. This implies a limit of applicability. cisms of the prohlems given in our paper. Concerning Problem 11. The section in which this prohlem the first part, our own experience suggests that the appears begins: "If two slightly soluble salts . . . are in capacities of second and third term chemistry students equ~librium with a solution." fall somewhat short of Professor Ricci's estimate. Problem 13. This should read: "At what pH mill Such students require considerable drill in applying zinc sulfide dissolve in saturated hydrogen sulfide solusimple algebra to chemistry. tion (1 atm.) to form a 0.1 M Zn++ solution?" As for the problems in our paper, it was not our inProblem 14. The solution to this problem indicates tention that students be given the over-all equilibrium constant expressions but rather that they work out that no material change in the concentration of sodium these relationships for themselves using the simple pro- hydroxide occurs due to its reaction with zinc hycedure outlined in our paper. Our main interest was to arOX1ae' COLLEGE DAVIDDAVID~ON show that many different problems occur, whether they be real or academic, in which this procedure may be BROOKLYN, NEWYORK
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