The QUANTITATIVE BALANCE in GENERAL CHEMISTRY MARSHALL A. GOFF State Teachers College. Lacrosse. Wisconsin
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EVERAL factors probably contribute to the fact that the quantitative balance is not used in geuera1 chemistry courses to the extent that it should be nsed. Large laboratory sections, lack of teaching assistants, little equipment in the line of balances and, not least, lack of time to put the freshman through the process of learning the weighing technic of quantitative analysis, and other reasons might be given for this condition. And yet no other laboratory procedure so concretely fixes in the mind of the student those fundamental chemical laws as work with the balance. Such exercises may well illustrate the laws of definite and multiple proportions, molecular and atomic weights, equivalent weights, valence, volume relations in solutions, etc. The following notes are given with the idea of showing that the above reasons for not using more quantitative experiments are not necessarilj. insurmountable and that the technic of the freshman balfmce is very simple. In the first place weighings to the nearest centigram are all that are required to attain the necessary accuracy in the exercises suggested above. If samples of a t least 1 gram are nsed an error of 1 centigram represents not more than 1 per cent. Balances of sturdy construction having a sensitivity of 1milligram per space may be purchased very cheaply. (They are listed a t less than thirty dollars.) Weights having the tolerance of class S-2 sell for less than ten dollars. With such apparatus weighings may be made to the third decimal place if desirable.
I n this laboratory the balances used have a sensitivity of less than 1 space per milligram, about 0.6 space; thus 5 milligrams move the point of rest about 3 spaces. The zero point of these balances is kept adjusted within 0.6 space of zero on the scale. Hence an assumption of zero on the scale as the true zero point involves an error of no more than 1 milligram. Again, instead of taking an odd number of readings on one side and an even number on the other, one reading only is taken on each side. Since the damping of swings due to friction averages about 0.5 space.the error involved in locating the point of rest is less 'than 0.2 space and since weights are taken by diierence these errors practically disappear. To make a weighing to the nearest centigram the student adds weights to the pan down to the 10 milligram weight that keeps the point of rest within 3 spaces of zero on the scale, that is, the number of spaces the pointer is moved by 5 milligrams. To simplify the instruction mimeographed sheets directing an exercise on the use of the balance are given out. Students are allowed to work in pairs. A demonstration is given by the instructor to the whole group, and individual instruction of less than five minutes to each pair. The weights themselves are checked out and in a t the beginning and end of each laboratory period. One or two students assist in the individual instruction. Those students not engaged in the quantitative experiment go on with the other laboratory exercises. I n this way six balances serve a section of twenty-four, half working on quantitative exercises a t one time.
