CHAPTER XX COMBINING WEIGHTS AND MOLECULAR WEIGHTS 117. The combining weight of zinc. From your results obtained in § 49 calculate the combining weight of zinc referred to hydrogen as unity. Repeat the experiment, substituting for the sulfuric acid a solution of hydrochloric acid, prepared by adding 1 volume of the laboratory acid (density 1.12) to 1 volume of water. Compare the results. 118. The combining weight of magnesium. Determine the combining weight of magnesium, referred to hydrogen as unity, by dissolving the metal in dilute hydrochloric acid and measuring the hydrogen evolved. About 0.5 g. of magnesium ribbon, prepared as directed in § 44, should be used. The hydrochloric acid is prepared by adding 1 volume of the laboratory acid (density 1.12) to 4 volumes of water. The apparatus used is shown in Fig. 27. 119. The combining weight of aluminium. Follow the same method as was used in the determination of the combining weights of zinc and magnesium. The hydrochloric acid used is prepared by diluting 2 volumes of the laboratory acid with 1 volume of water. 120. Determination of molecular weights. Molecular weight of chloroform. (The following experiment will be performed by the laboratory instructor. The students will answer all questions and make the calculations. Before performing the experiment, read over the directions and discuss the method until you thoroughly understand it.) Obtain from the storeroom the Victor Meyer apparatus shown in Fig. 49. This apparatus consists of two glass tubes, the smaller one of E which is suspended in the larger, as represented in the diagram. The liquid in the outer tube D is water. In the bottom of the inner tube A is placed a little asbestos fiber or sand. The graduated tube E has a capacity of about 50 cc. Arrange the apparatus as shown in the figure, except that the tube E is not brought over the exit tube C. Tightly stopper the tube A with a rubber stopper provided with a stopcock B, and heat the water in D to boiling; then regulate the flame so that a uniform heat is applied, the heat being sufficient to keep the water boiling. The air in tube A is heated by the steam, and some of it escapes through C, bubbling up through the water. While the heat is being applied, wrap the end of a fine platinum wire 5 or 6 cm. in length about the neck of the little glass-stoppered bottle F, suspend the bottle from the beam of the balance, and weigh it. Completely fill the bottle with chloroform, insert the stopper, carefully wipe the bottle to remove any of the liquid adhering to the outside, and reweigh. Now observe whether the air is still escaping from tube C. If not, open the stopcock B, remove the stopper and stopcock B, and, after loosening the stopper in the little glass bottle, hold the bottle in a vertical position with the platinum wire and drop it into the tube A. As quickly as possible insert the stopper and close B. Before any gas can escape, bring the tube E (which must be completely filled with water) over the end of the tube C and clamp it in this position. The bottle F drops to the bottom of the tube, and the chloroform, on account of its low boiling point (61°), is vaporized by the heat. The vapor formed expels a definite volume of air, which is caught in the graduated tube E. The heating is continued until the gas ceases to escape. D Ꭺ FIG. 49 |