Sidney P. Harris Bovside Hiqh - School Boyside 61, New York
I I
Determining an Empirical Formula A simple, rapid high school experiment
Early in high school courses and books, the Law of Definite Composition is stated and explained. Despite the best efforts of teachers and hooks, pupils generally do not realize that there are experimental derivations for this law. In courses which begin
Figure 1 .
516
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The modified balonce.
Jourwl of Chemical Education
with a study of atomic structure, there is little realization on the part of the pupil of 'the experimental basis for the knowledge of the proper empirical formula. Particularly does this present a problem when two elements can form two or more compounds. From the st,andpoint of the high school teacher of chemistry, the outstanding obstacles to the solution of these problems are the lack of suitable balances or money to buy them, the short laboratory period and the lack of good exhaust facilities. The obstacle of the costly balances mas overcome by modifying a type of balance suggested originally by the Physical Science Study Committee.' This modified balance, shoml in Figure 1, has a razor blade knife edge and aluminum pans made from frozen food containers. The total cost for making twelve such balances was about $1.50. A typical sensitivity curve for this balance is shown in the graph (Fig. 3). As a preliminary experiment in the use of the balance, the members of the class were given various lengths of wire and asked to determine the weight per unit length P.S.S.C. Lab. Guide #2, 1958, 8/29/58 Prelim. Ed. P160-1, 160-4, 160-5.
without the use of any instruments except the balance and a straight edge. The results were tabulated and a mean value of 29.3 mg/in. was obtained. The per cent average deviation was 0.6%. The wire was identified as being #26 and reference to The Handbook of Physics and Chemistry gave the value of 29.2 mg/in.; an error in the mean of 0.34%. The Experiment Use about 5 in. of X26 copper wire and bend it to assume a hairpin form. Weigh the copper wire. Support vertically an s i n . Pyrex tent tube and add to it about 10 g of powdered sulfur. Hang the copper loop from a hooked glass rod and support the rod irom the lip of the test tube as shown in Fiaure 2. The oopper loop shoula extend almost into the sulfur. The sulfur is heated strongly until one minute after the brown vapor has risen above the copper loop. The combustion in the vapor is usually visible. The heat source is removed and the apparatus is allowed to cool for one minute to prevent the oxidation of the Glass hook hot condensed sulfur by the air. The glass hook is lifted out of the test tube and the euprous sulfide hairpin loop is placed in a. clean and dry 6-in. test tube slightly inclined from the horNo. 26 copper izontal. The product is heated wire strongly for oneminute and the flame ismoved slowly toward the The oxidation mouth of the test tube to prevent of copper b y the inrush of air. Alternatively, sulfur vapor in a n 8-in. pyrex natural gas may be passed into test tube the test tube and then the cuprous sulfide may be heated to Sulfur remove the excess sulfur. Either procedure is satisfactory although the second demands the Figure 2 usual care to Drevent ex~losive burning. The product is then removed and weighed.
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Seven trials by high school students with this procedure gave a mean ratio of Cu/S of 3.90 with an average deviation of 1.8%. (The theoretical ratio is 3.95 which gives a percentage error of the mean of 1.3%.) Three further results using an ordinary student type analytical balance gave a mean of 3.94 for the
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5
10
15
Load in grams Figure 3.
Bolancerenritivity.
ratio of Cu/S. A modification of the procedure was tried by using a strip of copper 0.05 cm thick and approximately 5 X 1.25 cm suspended by means of a hole punched in the strip. Similar accuracy was obtained again in four trials. Very thin copper foil must not be used because the exothermic reaction causes peeling off of some of the product. Although superior results are obtained when using manufactured balances, the use of the inexpensive homemade balance gave sufficiently accurate results and taught the students a great deal about balance construction and theory. The experiment using copper wire is preferable for high school work because sufficient accuracy will be obtained if the length of wire is measured carefully and only the product is weighed. The empirical formula, Cu2S was derived from the data using the atomic weights of the elements. Student interest was increased by the fact that one of the two textbooks used states that the product is cupric sulfide. The method given seems to have advantages in accuracy and rapidity over the usual method where copper and sulfur are heated in a crucible. Every step in the experiment is visible and there is never a large amount of excess sulfur to drive off after lengthy heating.
Volume 37, Number 7 0, October 1960
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