1144
JOURNAL
oa CHEMICAL EDUCATION
JUNE,1929
THE MEASUREMENT OF CHANGE IN VOLUME ON SOLUTION
The following exercise, not commonly outlined in laboratory manuals, is included in the author's list of experiments for the course in elementary physical chemistry. It offersthe student training in the careful determination of the density of liquids, in the use of pipets, and at the same time gives him the opportunity of studying experimentally the phenomenon of change in volume which accompanies the mixing of certain liquids. The experiment has the advantage, if it may be called an advantage, of being easily and quickly carried through. The ordinary student should obtain fairly concordant results in one laboratory period of three or four hours. The directions call for the preparation of a series of solutions containing the same components in differentproportions. For example, the following has been found to be a convenient series. (1) 10 cc. methyl alcohol, 90 cc. water, (2) 30 cc. alcohol, 70 cc. water, (3) 50 cc. alcohol, 50 cc. water, (4) 70 cc. alcohol, 30 cc. water, (5) 90 cc. alcohol, 10 cc. water. Technical methyl alcohol (methanol) may be used. These volumes are conveniently and accurately measured with pipets, and the solutions may be prepared and kept in stoppered flasks or bottles. A visual demonstration of the volume change may be had by mixing the liquids in a 100 cc. volumetric flask having several gradations on the neck. The student should notice the relative temperature changes which*accompany the mixing. The solutions are then given time to come to room temperature, or, if desired, to the temperature of a thermostat. Meanwhile the densities of the alcohol and the water may be determined, the temperature a t which this is done should be that a t which the solutions will be when their densities are measured. For these measurements use may be made of any of the common methods for determining the density or specific gravity of liquids. The pycnometer method permits great accuracy but is relatively slow. The author has usually assigned the specific gravity balance, either of the Mohr-Westphal type or of the ordmary balance or chainomatic type. Careful workers with good instmments and a little practice can reproduce density determinations to within a few units in the fourth decimal place. Short range, carefully graduated hydrometers might also be used. The change in volume is calculated from the original volumes and the measured densities. Several slightly d i e r e n t methods of calculation are available. Some of these will be illustrated from actual observations on mixture number (3) of the above series. In forming this solution 50 cc. of methyl alcohol are mixed with 50 cc. of water.
(1) Percentage contraction calculated from change in density. 0.9973 g./cc. 0.7894 g./cc. 0.9241 g./cc. 50 X 0.9973 50 X 0.7094
Density of water. Density of alcohol. Density of mixture. = 49.8650 g. Wt. of water. = 39.4700 g. Wt. of alcohol.
+
49.8650 39.4700 = 89.3350 g. Wt. of mintwe. 89.3350 t 100 = 0.89335 g./cc. Density of mixture if there had been no contraction. 0.9241 - 0.89335 = 0.03075 g./cc.. Increase in density. 0.0375 X 100 s 0.9241 = 3.32%. Percentage contraction.
(2) Calculated from change in volume. 89.3350 i0.9241 = 96.672 cc. Volume of mixture. If there had been no contraction the volume would have been 100 cc. 100 - 96.672 = 3.32 cc. or 3.32%.
(3) Calculated on the basis of specific volume. 100 s 89.3350 = 1.1193 cc./g. Specific volume if there had been no contraction. 1 t 0.9241 = 1.0821 cc./g. Specific "01. of solution. 1.1193 - 1.0821 = 0.0372 cc./g. Contraction. 0.0372 X 100 t 1.1193 = 3.32%.
The following graph, constructed from a typical student report, will serve to show the type of results which may be obtained.
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0% 0 10 20 30 40 50 60 70 80 90 100 cc. methyl alcohol 100 90 80 70 60 50 40 30 20 10 0 cc. water
Other pairs of liquids may be studied in place of the one described. Acetone
