An ATOMIC WEIGHT DETERMINATION for HIGH-SCHOOL STUDENTS E. L. GUNN Robert E. Lee High School, Goose Creek, Texas
(1) This article describes a n experiment i n the determinution of atomic weights for high-school students; the need and purfioses of such a n experiment are pointed out.
(2) Certain criteria for a guantitatinre experiment to be done by high-school students are proposed. ( 3 ) A table of typical results obtained by students i s presented.
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HE teacher of high-school chemistry often feels a need for a simple experiment which will illustrate to the student the meaning of atomic weights. The atomic weight of some particular element may hecome a real and significant thing to the elementary student if he has had actual experience in the quantitative determination of the atomic weight of that element. To he suitable for the use of high-school students any experiment which proposes to he quantitative in nature should have a t least the following qualities. (1) It should involve a minimum of technic and experience on the part of the student. (2) The apparatus and reagents required should be those found in most high-school laboratories. (3) The experiment should not require more time for completion than the average laboratory period affords. (4) The method should be one which consistently gives fairly accurate results. (5) There should be no element of danger to the student in the performance of the experiment. Since atomic weights are usually expressed relative to the oxygen atom, i t seems that the synthesis of a binary oxide affords the most desirable type of experiment for the illustration of atomic weights. Methods of direct synthesis-magnesium oxide for example-are described in various laboratory guides. There are several objectionable features to the use of the direct method for high-school students; hence, we have resorted to an indirect method of synthesis. A number of laboratory manuals, high-school and college,' describe an indirect method of oxidizing such metals as tin, copper, and so forth, with .nitric acid. The writer has not encountered a manual which describes the method of indirect oxidation of copper as applied to a determination of atomic weights, however. Herewith is presented a description of such a method. A weighed quantity of coppq turnings in a crucible is dissolved in concentrated nitric acid, added drop by drop, until action ceases. The resulthg copper nitrate is slowly evaporated with a very small flame. Heat is further applied until the copper nitrate is decomposed into the black oxide of copper. The evaporation and decomposition should be done with care to avoid the loss of materials by spattering. The reaction between the nitric acid and the copper must be done in a fume hood or under some other condition where the oxides of nitrogen can be removed; many teachers may prefer decomposing the copper nitrate in a hood also. The black oxide of copper should be heated strongly to constant weight to insure complete decomposition of the nitrate. The crucible is then cooled to room temperature and weighed to an accuracy of one-hundredth of a gram.
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HOL~S "Laboratory . manual of general chemistry," 4th ed., The Macmillan Company, New York City, 1937, pp. 77-8.
The following are student values with the atomic weights of copper found. TABLE 1 Sludcnl
No.
Grants of c o p w Gvomr of CODPEI ALomi~waighl of used oxide formed co99ar
~
Avdogr (Accepted value
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65.5
63.6)
The following is a sample of the calculations done by the student. The data are those of student No. 10. Wt. of copper used.. . . . . . . . . . . 2.64 g. Wt. of copper oxide formed.. . .. 3.29 g. Wt. of oxygen used.. . . . . . . . . . . 0.65 g. Since Wt, of copper used :Wt. of oxygenused = Comb. wt. of capper (referred to 0 = 16) : 16 g. oxygen. By substitution, 2.64 : 0.65
:Comb. wt.of wpper : 16
Comb. wt. of copper = 16
x 2 . 6 4 = 64.98 0.65
(The student's value to one decimal place is 65.0)
Some objectionable features af this method are that the inexperienced high-school &dent has to handle concentrated nitric acid; besides, the nitrogen oxides are poisonous. By taking due cautions, however, these dangers can be obviated. Some advantages which the choice of copper in this method seems to possess are the following. (1) Copper turnings are inexpensive and would be more commonly used in the average high-school laboratory than such a metal as tin. (2) Copper turnings dissolve in nitric acid readily. (3) The combining ratios give the "true" and not the "apparent" atomic weight when copper is used. (4) Copper is suitable for demonstration and explanation of how the atomic weight of an element is found from its specific heat-based on the law of Dulong and Petit; such a demonstration gives the elementary student further insight into the quantitative aspects of atomic weight determinations.
