MODIFIED VICTOR MEYER APPARATUS FOR THE DETERMINATION OF MOLECULAR WEIGHTS
A diagram of the Victor Meyer apparatus for molecular-weight determinations is described and results of a typical determination are tabulated. The apparatus is constructed of regular laboratory stock and combines ease of manipulation with a considerable saving of space. Very small quantities of sample are requir~dfor a determination. The Victor Meyer method for molecular weight determination is a standard laboratory experiment in physical chemistry, ( I ) , ( 2 ) , (3). A weiehed amount of the substance with unknown molecular weight is vaporized, and the volume which it would have occupied a t the temperature of the room and under existing pressure is determined. The following apparatus was designed to supply the need of a regulation Victor Meyer set-up during the course of experimental work. Constructional details are shown in the accompanying figure. Dimensions are variable, the only requirement being that the inner test tube be of such dimensions that the vaporized sample shall not extend too far up the tube. Otherwise condensation of the sample will take place and vitiate resnlts. The volume of air displaced by the vaporized sample is collected in a graduated pipet immersed in a cylinder of water. To carry out a determination of vapor density with this apparatus the rubber stopper is removed from the top of the T-tube and the liquid in the outer tube heated to boiling. In ten minutes the temoerature will be approximately constant. The cylinder of water is then raised or lowered till the water level is a t the zero of the graduated pipet. The stopper is inserted in the T-tube and no change in the water level in the pipet is an indication that the temperature is constant. A sample of the substance (0.0200 to 0.0250 g.) is weighed into a
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VOL.8, NO.3
547
DETERMINATION OF MOLECULAR WEIGHTS
small stoppered bulb and dropped on sand or glass wool in the bottom of the vaporization tube. The impact together with' the increase in temperature drives the stopper out of the bulb and the substance vaporizes, driving out an equal volume of air which is collected in the pipet. The position of the leveling cylinder is now adjusted and the volume of the displaced air in the pipet is measured. Since the air is measured over water the pressure to which it is subjected is that of the atmosphere diminished by the vapor pressure of water a t the temperature of the room. Calculation of the molecular weight involves correction of the volume of displaced air to O°C. and 760 mm. pressure and finding the mass of that volume of vapor which will occupy 22.4 liters a t 0°C. and 760 mm. pressure. Convenient formulas for the calculation are given by Bigelow ( I ) , Walker (2), and Getman (3). Typical Determination of the Molecular Weight of Benzene (C&L,) Sample,
Volume of Vapor,
Temp.,
G.
Cm."
OC.
Waighl of
0.0228 0.0202 0.0148
7.45 6.68 4.90
20
Moiarulnr
Barometric Prrsrurr
M m . Hp 742.8
Corrected 725. 4
IVeiaht Obr.
77.1 76.2 76.1
Cal.
78.05
Av. 76.5
Literature Cited "Theoretical and Phvsical Chemistrv." The Ceutuw Co.. New York City, 1912, p. 165. WaLKpn. "Introduction to Physical Chemistry." Sixth edition. The Macmibn Co., New York City, 1910,p. 198. GETMAN, "Outlines of Themtical Chemistry," Fourth edition, John Wile? and Sons, New York City, 1927, p. 35. BIGELOW.
