A Device for Teaching Chemical Arithmetic HORACE G . D E M I N G University of Nebraska, Lincoln, Nebraska TUDENTS in elementary chemistry perhaps more S frequently fail or drop out on account of inability to grasp chemical calculations than for any other single
chemical equation which shows how many mols of the one correspond to any known number of mols of the other. reason. The most successful remedy we have found, When both substances are ionic or when they serve during years of experimenting with different methods, as oxidant and reductant, the number of equivalents is the annexed diagram which relates all the important or milliequivalents of the one is equal to the number of types of problems that are based on chemical equations, the other. Various short cuts soon become evident. points out logical sequences of steps to be taken, and For example, it is observed that the volumes of reacting helps train the student to explain clearly how he ar- gases are in the same ratio as the numbers of mols in rives a t his answers. The purpose last mentioned is the balanced chemical equation. deserving of much more emphasis than it usually gets. Chemical factors may be mentioned as a short cut The operations summarized in different parts of the from grams to y-rams, to be used chiefly by analytical diagram are explained in a number of successive lec- chemists, who may need to solve hundreds of such tures. Then a sheet is mimeographed that repeats the problems, all based on the same chemical equation. diagram eight times, with a few textbook references A slight modification of the diagram will provide for included. This provides a standard form for working problems based on engineering (English) units. eight problems. Students are directed to trace each problem through, The student enters the data given for substance A, setting down the result obtained in each step on the then shows what result he gets for each step until he way to the final result, then to check the work by tracarrives a t the answer, which may relate to substance ing i t backward, each former operation being reversed. A or to another substance B that reacts with or is re- Practically everyone becomes nearly infallible by the lated to A in a known chemical reaction. When he has time he has worked enough problems of assorted types found the number of mols of one substance, he may to till two or three record sheets. Then he abandons "cross over" to the other by referring to the balanced this crutch of his own accord. Problem no:
Balanced equation:
ml. (soh.) X g./ml. (density) X per cent/100
-
X factor B/A g. of A
g. of
_ A
X factor A/B
- M. W.
i22.4
liters gas (S. T. P.)
+
I
-
-
per cent/100 X g./ml. (density) X ml. (soh.)
M. W.
i22.4 mols of B c-- liters gas (S. T. P.)
+
Equation
-
liters gas (other conditions) norm'ality X ml.
Consult
nlols of A
B
X valence units (+ or -) in famula*
I 1 =
X valence units (+ or -) in famula*
meq. ----t equivs. of A t----tequivs. of B i1000
* Use change in valence when substance is oxidant or reductant.
+meq.
-- 1000
I
liters gas (other conditions)
-
normality X ml
