phenolphthalein and then slowly run in a solution of sodium hydroxide until the solution is neutral, as in § 69. If a solution of sodium hydroxide of known strength is used, it is evident that one may in this way determine the percentage of acetic acid in the vinegar. The laws of most of the states require a minimum of 4 per cent of acetic acid in all vinegars. 128. Saponification and the preparation of soap. Add 10 cc. of alcohol to 4 or 5 g. of cottonseed oil in an evaporatingdish. To the resulting mixture add about 1 cc. of a 50 per cent solution of sodium hydroxide in water. Evaporate carefully, stirring the mixture constantly until the odor of alcohol can no longer be detected. Write the equation for the reaction on the supposition that the oil is composed of palmitin. What remains in the dish? Add 75 cc. of cold water, stir well, and filter. Add a little of the resulting filtrate to test tubes containing solutions of calcium sulfate, magnesium sulfate, and sodium sulfate respectively. Explain the results. Account for the fact that soaps do not lather freely in hard waters. CHAPTER XV EQUIVALENTS AND MOLECULAR WEIGHTS 129. The hydrogen equivalent of zinc. From your results obtained in § 49 calculate the hydrogen equivalent of zinc. 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. 130. The hydrogen equivalent of magnesium. Determine the equivalent of magnesium 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. 131. The hydrogen equivalent of aluminium. Follow the same method as was used in the determination of the equivalents of zinc and magnesium. The hydrochloric acid used is prepared by diluting 2 volumes of the laboratory acid with 1 volume of water. 132. 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. 45. This apparatus consists of two glass tubes, the smaller one of |