The Perceived Purpose of Quaniltatlve Analysis
Llterature Clied M. L. Physica-Chmical Qurnlitiea and Units, 2nd sd.; Royal in& of Chemistri; london. 1971.
1. &O!mhan,
2. McOlaahao, p4. 3. Ouggenheim, E. A.Phil. Mag. 1942,33,419.
Ronald L. Rich 112 S. Spring St. Blufnon, OH 45817
To the Ed;tor:
Ronald Rich discusses my use of descriptive units [1987, 64,1911 in the prohlem of calculating the concentration of a 96% sulfuric acid solution of a known density. This problem calls for an understanding of the concepts of concentration (molarity), molar mass, and density. In the symbols recommended by IUPAC, these concepts are defined by c = nlV M = mln p = mlV
(1) (2) (3)
The expression "96% sulfuric acid" is ambiguous hut is taken here to mean mass of sulfuric acid X 100 mass of solution From (I), From (Z),
c(H2S04)= n(H2S0,)/V(solution) n(H2S04)= m(H2S0,)IM(H&0,)
To the Editor:
I should like to express a differing point of view from that of Hirsch (Provocatiue Opinion, May 1987) on the suhiect of the i n t r o d u c t o ~auantitative analvsis course. ~" My difference lies, I think, in the perceived purpose of the "auant" course. In mvview.. its nuroose . . is to reinforce sliehtl i s o m e notions of equilibrium theory and to give a q&kscan overview of the field of analytical chemistry; hut its primary usefulnesslies in its ability to induce in the students habits of precise, careful work in the lahoratory. In this course, as traditionally taught, extreme emphasis is placed on the attainment of precise, accurate data by means of systematic application of greatly simplified instiuctions. For the professional analyst, sampling is vitally important. and knowledee of what methods are available and of what their limitations are constitutes asine qua non. But, as Professor Hirsch points out, this course is "...the only course in analytical chemistry most students ever see". By far the majority of our students do not become analysts, and mainstream analytical concepts and techniques will not prove useful a t this level. By all means, these concepts and techniques should he t a u g h t h u t in upper-division courses intended for analysts.
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Myron Colllns Saint Mary's College Mmga. CA 94575
(4)
(5)
Now we note from the data From (3),
m(H2S04)= 0.96 m(solution) mfsolution)= solution) X V(solution)
(6) (7)
We can now substitute known quantities for n in (4). From (5), (6), and (7) we obtain:
Slgnlflcant Flgures and Error Propagation To the Editor:
- 0.96 X 1.84 g mL-' 98.0 g moT1 = 0.018 mol mL-'
The relation (8)is obtained from algebraic manipulation of (I), (2), and (3) and should not be memorized. On the other hand, (1). (2), and (3) should he memorized, together with the meaning of the symhols. Omission of the 0.96 occurs when the student fails to distinguish between the mass of sulfuric acid and the mass of sulfuric acid solution. The advantage of this procedure is that the relationships used in the solution are stated exolicitlv in terms of ohvsical quantities. The reader does not have tddeduce whic'h physical quantities are involved by examining the units. McGlashan ( I ) said that i t would be a logical procedure to assign different dimensions to lengths of desks-chemical bonds, etc, and presumably, for concentrations of sulfuric acid and of hydrochloric acid. But, although this may he a logical possibility, no decision has been made by IUPAC or other international body to introduce such a procedure. There appears to he no compelling reason for avoiding the IUPAC conventions described in McGlashan's hook.
Schwartz 11987.64.471-4721 is wrong to sav that 125 mL sysiembtic erroiof *5%.-whenthe manufacturer labels its flanks like this it is telline us somethine about the random error of the process by which the flaiks are made. That "something" may be one standard deviation, or two, or a confidence inierva$ if we asked, it could, presumably, tell us what its criterion is for labelling the flasks as i t does. In any case, the signifies an experimentally determined random error, not a systematic one. When we measure carefully the actual volume of one specific flask we can indeed establish the systematic error for that flask and the random error of our series of measurements. However, until we do so, the f5% remains a random error, which must he assumed whenever we use another, uncalihrated flask. Steia (1)is right in asserting that the correct wav to calculate random er;or is to use standard deviations in ihe appropriate formula for error propagation. The problem is, as he le deviations implies, that if we do not h a ~ ~ r e l i a hstandard our estimate of random error is only a rough approximation. Under these circumstances, "the rule of thumb" (2) gives a reasonable answer even in the worst cases.
Llterature Clted 1. MeGlaahan,M.L.PhyaicoehamicolQuontitierond Chemhtm: 1971.
LRerature Clted 1. Steig. S.J. Chem.Edue. 1981.61,471. 2. Schwartz, L. M.J. Chom. Educ. 1986.62.693-697
Units. (Znded.), RoyalInstitutcof
H. R. Kemp Vnlverslty College Australian htfence Force Academy Campbell. ACT 2600
272
Journal of Chemical Education
f 5% means
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Allen C. West Lawrence University P. 0. Box 599 Appleton. WI 54912
