An IMPROVED FORM of VAPOR DENSITY APPARATUS JAMES M. HENDEL
AND
OTTO OCHSENREITER
Hunter College of the City of New York, New York City
A n improved form is suggested for the Victor Meyer vapor density apparatus, including a simple and "sure fire" breaking device. Using this apparatus and following certain precautions, we haere determined m o k u l a r weights with a precision of two per cent and a great gain i n dependability. The sources of error have been investigated and for the most part eliminated. Unknown liquid mixtures are recommended as the best test of student performance.
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HE determination of vapor density by the method of Victor Meyer is an experiment included in practically all courses in physical chemistry. I t is of recognized value in giving the student experience i n handling gases and applying the gas laws to some definite end. Yet this classic experiment proves a stumbling block for most students. With a view to removing most of the difficulties a new form of apparatus was devised which bas certain advantaees " over the older types and gives more consistent results. The apparatus shown in diagram A consists of a pyrex teHt tube 400 X 50 mm-serving as the steam jacket for an improved vaporization tube. Since it is the iacket that is most often broken. the test-tube form is agreat advantage, being more rekitant to heat and mechanical shock and less expensive to repl*ce. The intube is so that practically all of i t is exposed to the steam, yet because of the spiral it provides a comparatively great distance for the vapor to travel before it reaches the exit. In this form, the distance from the bottom of the bulb, where the vapor is formed, to 52 cm. for the to the side exit is 126 cm. older form and 67 cm. for the Weiser modification.' The spiral tube is fitted into the rubber stopper so that it is immovable and may be lifted out of the jacket with the stopper. b hi^ latter also carries a g.mm. glass tube about 30 crn. long, to serve as an air condenser for the steam. glass tube and plunger device in diagram B is arranged to hold the bulblet containing the liquid to be vaporized in the cooler part of the apparatus until temperature equilibrium is established, when the plunger is operated to break the bulblet. ~tis well known that the bulbs usually do not break if simply dropped into the vaporizing tube, and if they do, the air is forced p
WEISER,3. Pkys. C k m . , 20, 532 (1916).
Over too quickly to be collected. So that the liquid will distil gradually and thereby displace the air slowl~,i t is necessary to keep the likuid cool and to break only the capillary neck of the bulb. Other breaking devices have been tried which involve loosening the stopper O r pulling up on i t to break the bulb. For the skilled operator these are satisfactory, but the average student either fails to close the tube tightly or to insert the stopper to the same depth as jt was before breaking the bulb. A difference of 1mm. ln the depth to which the stopper is inserted causes an emor 0.25 cc. in the air collected. The bulblet should be blown to hold about 0.1 gram of liquid; an outside diameter of 6-8 mm. is satisfacto^. Its stem should be a capillary not over one mm. outside diameter and bent a t an angle to fit the holes (2-mm. diameter) in the glass tube from which i t is hung. For ease in breaking, a fine stem is an advantage, but if the bore is too small the vaporization may be so slow that diffusion of the vapor may occur and too little air be displaced. The overall length of the bulb may not be more than 40 mm., as it must hang in the cooler Part of the tube. This Pa* of the tube may attain a temperature of 35'C. and if the liquid be very volatile it may spray out when the bulb is broken and invalidate the 533
experiment. For the same reason the bulb should not be entirely filled so that if appreciable expansion occurs on heating the capillary will still be empty. If it is desired, a small water jacket may be fitted over the projecting part of the vaporizing tube and a current of cold water used to keep the liquid in the bulb well below its boiling point. Keeping this part of the tube a t a constant temperature also aids in securing the pressure equilibrium necessary for the success of the experiment. Equalityof pressure before and after vaporizing the liquid can only be maintained if the temperature of the inner tube is kept constant. This condition requires that the water in the jacket be boiled steadily throughout the preliminary heating and the vaporization, that none of i t should be boiled away, and that the tube should not touch the boiling water or the sides of the iacket. Usuallv ten minutes' heating suffices to bringthe air in the vaporization tube to 100°C. and in another five minutes equilibrium is estiblished throughout the system. After the bulb is broken the vaporization is ordinarily complete in one minute for ether, five minutes for chloroform or methyl alcohol, and fromfive to ten minutes for benzene or carbon tetrachloride depending on the amount taken. In any case the heating should not be prolonged unnecessarily lest diffusion occur. The displaced air is collected in a eudiometer over distilled water which has been saturated with air. This latter precaution is necessary because 50 cc. of water a t room temperature can dissolve 2 cc. of air. The air in the eudiometer will have been cooled in its passage through the water so that as soon as the last bubble has come over, the eudiometer may be disconnected and the temperature, pressure, and volume of the displaced air may be measured. The pressure of the air in the eudiometer is given by the equation 2~~
the pressure measurement. For example, fanning the thermometer gave a value of thirty-five per cent for the relative humidity compared with a value of fortyfour Der cent without fanning. At 26' where the value I'bo - h of F is 25.7 mm., the term -F equals 16.7 mm. 100 for a humidity of thirty-five per cent and 14.4 mm. if forty-four per cent is used. The differenceof 2.3 mm. is ten times the probable error in the barometer reading. Neglecting to ventilate the thermometer may therefore introduce an error in the net pressure P equal to 0.3 or 0.5 per cent depending on what values of h and F a r e applicable. The molecular weight of the liquid is calculated by the equation ~
~
-
where Bois the barometer reading corrected for temperature and latitude, AL is the difference in water levels inside and outside the eudiometer expressed in millimeters, h is the percentage relative humidity of the laboratory air, and F the aqueous tension a t the temperature of the water in the eudiometer. The air in the eudiometer is presumed to be saturated with water vapor since it comes over bubble by bubble. The full value of the aqueous tension F, however, is not deducted from the total pressure but a certain fraction of it, loo - F as suggested by Evans2 The 100 humidity is determined by the wet-dry bulb thermometer method. Considerablevariation in the value of h results if the thermometers are ventilated or not, and this uncertainty in h produces the main error in EVANS,I.Am. Chcm. Soc., 35, 958 (1913).
where W is the weight of the liquid, V the observed volume of air, T its absolute temperature, and P the pressure found from equation (1). In the experiments tabulated below the observational errors range from 0.1 to 0.3 per cent in W, 0.1 to 0.5 per cent in V, 0.1 to 0.3 per cent in T and about 0.3 per cent in P. The maximum error in M should therefore be 1.4 per cent and the probable error 0.7 per cent. Considering the simple gas law to be valid to one per cent which affects the value of the constant 22,400, the total error in M may be as much as two per cent. Discrepancies greater than this must be attributed to leakage or faulty vaporization, both being-apt to cause high values for the moleqular weights. .. Table 1 shows the results obtained with the new vaporizing tube, using reagent grade liquids, the chloroform having been redistilled after drying over lime. The last two values for chloroform show that lame s a m ~ l eweights lead to high results. This may be due to thk longer time require> for vaporization with consequent chance for diffusion, or because the volume of the vapor formed a t 100°, (72 cc. and 85 cc., re-
-
-
T*BLB
1
M ,obr. -74.3 77.2
AT. 75.8 152.7 147.5 161.0
Av. 153.7 78.2 82.5
-
Av. 80.4 120 124 122 121 117 116 119 125 (132) (130)
-
Av. 120.5
119.4
1.1
spectively) exceeds the capacity of the bulb, 70 cc. Althouab the s ~ i r apart l of the tube offers an additional 45 cc. o? available space, yet it is apparent that diffusion and condensation have taken place. The values found for the other liquids show that two Per cent is the Precision to be expected of this apparatus, as the average deviations and the differences between the observed and theoretical values are usually of this magnitude. In this laboratory it is the practice to issue unknown liquids or mixtures of liquids to students. With a view to testing the procedure as it would be used in regular class work, one of us filled and sealed the bulbs with liquids entirely unknown to the other operator who then ca-med out the rest of the experiment.- The results are shown in Table 2.
Av.
2 ICCL)
62.1 96.2 112.0
80.5 161 146 160
78.0
2.5
lo) Mixture of CHIOH and CHCb 1:l by volume lb) Mixture of CHIOH and CHClr 1:2 by volume
The "molecular weight" of a mixture may be calculated from the equation
wland are the actual weights of the two pure their respective molecular liquids, M~and and is the actual weight of the mixture of average molecular M. For examole Liauid Number in Table 2 was a mixture composed of one volume of methyl alcohol to two volumes of chlorofom. The densities being 0,792 respectively, wl = 0,792 g,, w2 = 2,978 g,, and and = 3.770 grams. T J , ~ calculated value for the umolecular weight" therefore is
w
w
Thus by choosing suitable liquids, mixtures can be prepared for class use which are truly "unknown" since they cannot be analyzed simply by their odor. Methyl alcohol (B.P. 66") and chloroform (B.P. 61°) may be mixed in various proportions to give "molecular weights" ranging from 32 to 119. Similarly acetone (B.P. 57") may be mixed with chloroform to give values between 58 and 119. These liquids are recommended because of their close boiling points and widely divergent molecular weights. The vapor pressures being approximately the same, there is little tendency to fractionate on standing. In filling the bulbs with suction, however, it is advisable not to prolong the evacuation unnecessarily. The use of such mixtures serves as a challenge to the student as well as a test of individual performance. With the procedure herein described, the results with classes for the past two years have indicated an increase in precision and considerable gain in consistency over the apparatus heretofore used.
