AN EXPERIMENT TO ILLUSTRATE THE LAW OF MULTIPLE PROPORTIONS JOHN C. BAILAR, JR., UNIVERSITY OF ILLINOIS, URBANA. ILLINOIS The experiment which is used in most beginning chemistry courses to illustrate the Law of Multiple Proportions depends upon the thermal decomposition of potassium chlorate and potassium perchlorate. The aperiment may be outlined as follows: A test tube containing a little manganese dioxide is carefully weighed, and about a gram of potassium chlorate is placed in it; after determining the weight of the chlorate by another weighing, the oxygen is expelled. A third weighing enables one to calculate the weight of oxygen that is combined with a definite weight of potassium chloride in potassium chlorate. The experiment is repeated, using potassium perchlorate, and the ratio of the two amounts of oxygen that are combined with a fixed amount of potassium chloride is calculated. There are several disadvantages to such an experiment: (1) it requires six weighings, which are very time consuming for a beginner; (2) the student is apt to become so involved in the calculations that he loses sight of the object of the experiment; (3) small experimental errors may lead t o the ratio 4 5 , or 2:3, instead of the correct ratio, 3:4. Another experiment which has been proposed to illustrate this law depends upon the reduction of the two oxides of copper. This leads to the simplest possible ratio, 2 :1, but does not reduce the number of weighings or shorten the calculations. The difficulty in obtaining pure cuprous oxide is a serious disadvantage. This article describes an experiment which necessitgtes only three weighings, greatly simplifies the calculations, and involves the simple ratio, 2 :1. The following directions have been found satisfactory. In a dry hard glass test tuhe place about one gram of cupric bromide, and weigh the tuhe and contents accurately. Clamp the tube in a position slightly inclined to the horizontal, and fit it with a one-holed stopper (either cork or rubber) bearing a glass tube which dips into a flask containing a little water. Tlie end of the tuhe should not dip below the surface of the water, but should end about one inch above it. Heat the cupric bromide in the tuhe, gently at first, then more strongly, until bromine fumes are no longer evolved. During this operation it is necessary to warm the upper portion of the tube somewhat, in order to prevent the condensation of hromine. When the tube has cooled, remove the stopper. Tilt the tube so that any hromine fumes which remain in it may escape, and weigh the tube and its contents. Now pour 2 cc. of concentrated nitric acid into the test tuhe, replace the stopper and delivery tube, and boil the liquid gently until all of the acid has evaporated, and the residue is dry and black. Replace the one-holed stopper by one having two holes; through one hole should pass a tuhe reaching a little more than half way to the bottom of the test tube, through the
other a tube which extends not more than half an inch beyond the inner surfaceof the stopper. Connect one of these to a hydrogen generator; when a test shows that all air has been expelled from the apparatus heat the material in the test tube until it is completely reduced to metallic copper. Allow the tube to cool, while the stream of hydrogen is still passing through it, and weigh it again. The data may be recorded in the following manner: 1. Weight of test tube and cupric bromide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weight of test tube andcuprous bromide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Weight of test tube and copper.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Weight of bromine in the cupric bromide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Weight of bromine in the cuprous bromide.. . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Ratio of the two amounts of bromine (4 divided by 5 ) . . . . . . . . . . . . . . . . . . 2.
This experiment has been submitted to a group of sixteen students in general chemistry a t the University of Illinois-three of them obtained very unsatisfactory results due to faulty weighing or spilling part of the sample; the other thirteen obtained ratios varying from 1.95 : 1 to 2.01) : 1, the average being 2.03 : 1. The directions as given to this class specified that the hydrogen delivery tube should reach nearly to the bottom of the test tube. In following these directions most of the students allowed the copper oxide to come in contact with the delivery tube, to which a little of it adhered. This error, which accounts for the high results obtained, may be avoided by following the directions given here. Some of the students completed the experiment in a twg-hour laboratory period; most of them, however, did not quite finish it in that time. The time required for the completion of the experiment may be somewhat shortened if adequate ventilating facilities are provided for, in that case, the bromine and nitric acid fumes may be allowed to escape into the hood. It will be seen that by weighing the empty test tube, the weight of copper may be found, and the experiment may be made to illustratethe law of combining proportions as well as the law of multiple proportions; if the tube is weighed after the ignition of the nitrate t o oxide, the combining weights of copper and bromine may be calculated in terms of that of oxygen. It is difficult t o introduce these additions, however, without obscuring the main point of the experiment. The author wishes to express his indebtedness to Mr. Julius White for conducting the class experiments, and for several valuable suggestions.
