QUANTITATIVE EXPERIMENTS in ELEMENTARY CHEMISTRY 111. The Law of Jfultiple Proportions R. D. BILLINGER Lehigh University, Bethlehem, Pennsylvania
D
ALTON'S discovery (1802) of the law of multiple ratios was an important step in his theory of chemical combination. His reported researches substantiating this law were based (1) on analyses of nitrous oxide (laughing gas) and nitrous gas (nitric oxide); (2) combinations of nitric oxide with oxygen; (3) analyses of marsh gas and ethylene; (4) consideration of the known composition of the oxides of carbon. These data were cited in support of the existence of a simple numerical relation between the diierent quantities of one substance, A, combined with the same quantity of another substance, B. Wollaston in 1808 had independently discovered the law through his study of acid salts. He showed that the law applies not only to elements, but also to compounds which react with one another. The experiment outlined below shows how the law of multiple proportions applies to normal and acid salts, e. g., sodium carbonate and sodium bicarbonate. It is desired to compare the quantities of carbon dioxide liberated by the action of hydrochloric acid on equivalent quantities of the two salts. This is the same as if we were comparing the varying quantities of carbonic acid which react with the same fixed weight of soda. Two methods are proposed.
way of the shorter tube. Allow sufficienttime for the completion of the reaction, shaking gently to hasten the action. Now reweigh the flask and obtain by difference the weight of COz evolved. Empty and thoroughly cleanse the flask of any acid. Place in it the cooled NazC03 obtained by heating NaHCOa as described above. Determine the COz content as before. Compare the weight of COzevolved with that previouslv obtained. B.
VOLUMETRIC PROCEDURE
The necessary equipment consists of a 50-cc. graduated tube, 2' of I/," rubber tubing, an 8-oz. widemouthed bottle, one small glass vial, one 4" funnel, one No. 7 rubber stopper (2-hole), and one No. 00 rubber stopper (1-hole), and the requisite amount of glass tubing. Assemble as in Figure 2. The bottle, the graduated tube, and funnel should be attached to an iron stand as shown. Fill the graduated tube (B) and the rubber tube (C) with a saturated salt (NaC11 solution. Weigh on a quantitative balance two samples of sodium bicarbonate of exactly 0.15 g. each. Transfer one sample to a porcelain crucible and heat to a dull red for half an hour. While this is proceeding, place the other sample A. GRAYIMETRIC PROCEDURE in the bottom of the 8-ounce Procure an Erlenmeyer flask of 125 cc. capacity, a bottle (A). Carefully lower the No. 4 rubber stopper (2 hole), a screw clamp, two short glass vial (D), which should be lengths of glass tubing (about 3" and 5", respectively), about half full of hydrochloric and 1" of rubber tubing. Assemble as shown in Figure acid (1:4), into the bottle. Do 1. Each piece of tubing is constricted to an inside not allow acid to strike the salt diameter of about 1 mm. at the indicated sample until all connections are c ends (A and B). air-tight. Insert a glass plug (E) Weigh, on a quantitative balance, two into the second hole of the No. 7 samples of sodium bicarbonate of exactly stopper. Adjust the levels of the 0.25 g. each. Transfer one sample to a liquid in the graduated tube and porcelain crucible and heat to a dull red funnel until they are of equal for half an hour. (Care should be taken height; thus insuring the presa t this point not to fuse the sample by sure in the flask to be equal to excess heat, nor leave any unchanged bi- the barometric pressure. kecord carbonate by underheating.) The other the level in the graduated tube. FIGURE2 sample is placed in the flask. Carefully Now gently tilt the bottle so the draw into the lon~ertube sufficienthydro- HCI will run from the vial onto the sodium bicarbonate chloric acid (1:4) to fill it. ~ G h t e nthe clamp ( c j and sample. The Coowhich is liberated will be recorded weigh the assembled apparatus, containing the acid in the graduated tube. When the action has ceased, and the sample of sodium bicarbonate. adjust the levels to the same height and read the volRemove the flask from the balance and partially ume. The diierence between the initial and final readopen the clamp, thus adding acid to the flask. Carbon ings will give the volume of COz liberated at the dioxide is evolved by the reaction and escapes by existing pressure and temperature. 303
The sample which has been heated is now m l e d in a desiccator aud then transferred to the bottle. Proceed as before alld measure the volume of CG evolved. Compare the volume with that previously obtained. If the pressure and temperature conditions have not appreciably changed, the ratio of the two volumes, as measured, is also a ratio of the weights of COe evolved.
BIBLIOGWHY
(1) FREUND.IDA. "The study of chemical composition." Cambridge University Press, London. 1904, p. 159. (2) Em-.
(3)
F.. Z.a d . Chem.. 57, 65-71 (1918).
R., "Textbook of inorganic chemistry." The Macmillan Co.. London. 1930, p. 96.
P ~ ~ N G T O NJ.,
