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4 t& New England Association of ChemBt C $4
W. 1. Masterton and T. R. Williams
University of Connecticut Storrs
General Chemistry Laboratory
A cork stopper is inserted in the top of the hulk which i. then weighed ta rt-0.001g. The stopper is removed, a few milliliters of liquid added, and the cap slipped into place. The apparatus is damped in a water bath so that the water level is s t the narrow section of the hulh. The water is boiled eentlv for five minutes after all the liquid appears to have veen vilatiiized. The cap is then slipped off and the cork stopper quickly inserted. The flask is cooled to room temperature and the stopper removed momentarily to allow air to enter. The stopper is reinserted; the bulb is wiped off and weighed, and the molecular weight calculated.
Diffusion of vapor or steam through or around the cap is negligible unless heating is extended for more than 15 minutes after the liquid has been volatilized. A small amount of vapor may escape while the cap is being replaced by the stopper. So long as this exchange is carried out rapidly, the loss is insignificant. There is no tendency for vapor to escape when, the stopper is removed momentarily since air flows into the partially evacuated bulh at this point.
Two or more determinations can he carried out in a 3-hour period. The apparatus is easily cleaned by evacuating with a water aspirator. The bulbs are quite sturdy; 300 students using 40 bulbs over a one week period broke only one of them. 8 mm. DlAM.
lOmm.DUM.
BULB
Special apparatus.
The accuracy and precision of this experiment were checked with a class of 600 students using 5 liquids boiling from 60-84°C as unknowns. Half of the students used the apparatus shown in the figure; the others used an Erlenmeyer flask fitted with a cork stopper wrapped in aluminum foil. The molecular weights reported were corrected for deviations of the vapor from ideal gas behavior and for the air displaced by vapor in the final weighing. With carbon tetrachloride, the median molecular weight using the special apparatus was 156; the conventional apparatus gave a value of 150 (theoretical = 156). The poorest results were ohtained with the lowest boiling liquid, isopropyl . .. bromide, which gave medians of 119 and 114, respectively (theoretical = 123). The comparative precision of the two methods is indicated by the fact that 54% of the molecular weights obtained with the special apparatus were within *3% of the median while only 18% of those obtained~ using Erlenmeyer flasks this ~ ~ ~ were within i range. '
RANDALL, D. L., J. CHEM.EDUC.,31,297 (1954). CAMPBELL, J. A., AND STEINER, L. E., "Laboratory Ex~eriments in General Chemistry," The Macmillan Co., New York, 1955, p. 57. 3 sIENKO, M. J., AND P ~R.A.,~~ ~ ~ ~chemistry," , ~ McGraw-Hill Book Ca., Inc., New York, 1958, p. 79.
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Journal o f Chemical Education
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Vapor Density Apparatus for
The apparatus commonly used in general chemistry to determine molecular weights of volatile liquids consists of a flask fitted with a cork stopper equipped with a capillary escape tube. The cork may be wrapped in aluminum foil t o reduce absorption of vapor. Modifications of this apparatus which avoid the use of a stopper have been described.', 2, An error which seems to have received less attention is caused by evaporation of condensed liquid while the flask is being cooled and weighed. The apparatus shown in the figure is designed to eliminate errors due to absorption and evaporation. The bulb is made by dra,wing out the neck of a 200-ml Pyrex flask and sealing it to a piece of glass tubing. The tube is compressed a t one point to form a ring so that the hulh can be suspended from the arm of an analytical balance. The cap is made by sealing a piece of capillary tubing to a piece of Pyrex tubing just large enough to slip over the top of the bulb. The volume of the bulh is determined and marked on it.
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