Harry J. Svec Iowa State University Ames and Norman C. Peterson Polytechnic Institute o f Brooklyn Brooklyn. New York
II I
Semi-Micro Vapor Density Apparatus A rcmid technique
The measurement of vapor molecular weight by measuring pressure rise in a constant volume system was first suggested by Blier and Kohnl and Lnni~deu.~More recently Christian, Affsprung and Liu3 have determined molecular weights by pressure rise in an evacuated system, using moderate temperatures. We have developed a very rapid technique to determine the molecular weight of 5 to 15 of a volatile compound with a precision of about *2yo',. The method is adaptable for use in the undergraduate physical chemistry laboratory. Apparatus
The apparatus is shown in the figure. The tube has a volume of about 50 ml and is evacuated through the O-ring sealed Teflon stopcock4by a mechanical pump.
remove adsorbed gases, and tilting the tube so that the mercury runs into the U-tube. The apparatus is then arranged as in the figure with mercury above the glass frit. A 50-p1 syringe6with an integral stainless needle with the tip ground flat is used to introduce samples. Procedure
The tube is evacuated through the stopcock until the mercury levels in the manometer are even, and then the stopcock is closed. Only glass, Teflon and mercury are exposed to the sample vapors during the measurement. The syringe is filled past the calibrating marks and the plunger is removed. With the syringe held in the vertical position, the liquid slowly drains from the needle. The meniscus can he stopped a t a calibration mark by placing the needle tip under the surface of the mercury above the frit. When the tip is touched to the fritted glass the liquid flows into the measuring tube and vaporizes. The flow is stopped a t another calibration mark by simply withdrawing the syringe. A sample of 15 pl is used in a single determination. An unknown substance should be tested with smaller samples to demonstrate that the saturated vapor pressure is not the quantity being measured. The volume of the apparatus can be determined by measurements on reagent chloroform. A single pressure measurement requires about 5 min. Results
The manometer is filled by adding a measured amount of mercury to the large tube, evacuating the apparatus with the top section sealed with a rubber stopper, flaming the U-tube gently with a Bnnsen burner to Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. ' BLIER.0..AND KOHN.L. Monatsh 20. 505 (1849). ' LUMSDEN,' J. s.,J . c k . SOC.~ 3 , 3 4 2ii903j. "CARISTIAN, S. D., AFFSPRUNG, H. E., AND LIU, C.,J. CAEM. EDUC. 40,323 (1963). "Lab Crest" l'/& mm needle valve, Cst. 795-609-0114, Fisher and Porter Co., Warminster, Pennsylvania.
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Journal of Chemical Education
The volume of vapor is given by the ideal gas law expression and should be constant for a variety of nonassociating vapors, except for small changes in the manometer mercury level. The pressure rise was measured for 15 pl samples of a variety of volatile liquids and the results were used to compute the volume for comparison. The results are summarized in Table 1, in which pressures are average values for a series of determinations. The range of calculated volumes is 2.2 nd or about 4.4% for the liquids examined. The error in applying a handbook value of a density reported a t 20' for a 25" measurement is as much as 0.5%. The maximum volume variation caused by changes in manometer level is estimated as 0.25 nd, about 0.5%. Density Determination
Determination of the molecular weight of an unknown sample requires a measurement of the liquid density. The Hamilton Co., Whittier, California, Microliter Syringe No. 1705-N.
Table 1. Pressure Rise and Calculated Volumes in V a ~ o r Density Measurements. Sample 1 5 ~ a1 t 25'C.
Pressure, torr
M
Density I )
(g
Volume (ml) Calculated
Acetone Benzene Chloroform Cyclohexme Ethyl bromide Ethyl formate Diethyl ether Ethanol Triehloroethylene a For a 5 densed.
sample. A 15 rl sample of ethanol partly con-
The density of several samples was measured by fillimg the 50 pl syringe with the liquid, removing the plunger and adjusting the meniscus to the 50 J mark by touching a tissue to the tip. The syringe was weighed on an analytical balance to 0.1 mg. The sample was removed by inserting the plunger, the liquid wiped off the tip and the syringe reweighed. The following densities (g/ml) were obtained a t 25°C: benzene 0.874, 0.876; chloroform 1.486, 1.480; diethyl ether 0.706, 0.710; water 0.996. The densities determined by this method are a11 within 1% of the accepted values, an accuracy sufficient for the molecular weight determination. The rate of evaporation from the unsealed syringe was remarkably slow, and no difficulty was experienced in the weigh'mgs.
Volume 42, Number 6, June 1 9 6 5
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