Julie J. Kaya and J. Arthur Campbell Harvey Mudd College
Clarernont, California 91716
Molecular Weights from Dumas Bulb Experiments
W e have used a Dumas bulb experiment m ~ dstudent unknowns for some years in our freshman course with moderately good results. In an attempt both to improve the results and make the experiment less cookbook, we have determined the effects of various variables using as a Dumas bulb a 250 ml Florence flask whose orifice has been necked down and sealed to about 5 em of 1 mm capillary tubing. The capillary is then bent at right angles to the vertical in order to fit easily on a balance (see figure). Tables I, 11, and I11 (see appendix) are given to the students. The three substances were chosen to determine the effects of varying volatility. The legends for the Tables are phrased in terms of directions just as we give them to the students. We feel that this addit,iou of experimental data related to experimental uncertainties has greatly improved the experiment and intend to design more such experiments. These writeups will give the students actual data on the basis of which they must design their own techniques rather The Dumor bulb than the usual approach of saying: "Heat 10 ml of unknown in a Dumas bulb for five minutes. Remove the bulb, let cool, and weigh." Our student interest and involvement in the experiment is appreciably greater and results are more precise and accurate since they can make the decisions intelligently. We also give the student Table IV (see appendix) exactly as shown here to save him the time of thumbing t,hrough the handhook tables. He can use an approximate molecular weight with an accurate refractive index1 to make an identification. He then uses the vapor pressure data to calculate a more accurate molar weight and thus to identify the unknown with certainty. An observant student can also use the boiling point of his sample to eliminate from consideration some substances listed in Table N. The student uses his best value of the molar weight and molar volume at his experimental conditions to We are most appreciative of a grant from the NSF Science Course Irn~rovement Proeram which sunnorted this work in .. development of laborator; experiments. See GEORGE, S., AND CAMPBELL, J. A., J. CHEM. EDUC.,44, XI3 . . 11967). - . ,CAMPBELL, J. A., J. CIIEM.Ennc., 42.488 (1965).
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Journol o f Chemicol Education
calculate: the ideal gas constant and, hence, the deviation of his gas from ideality; the value of a in Van der Waal's equation, assuming b = V J 3 (he assumes V , = V , = M / p ) ; the critical pressure and temperature using the Van der Waals a and b values, and the equations P,= a/27b2, T, = 8a/27bR. These calculations of R, a, P,,and T,teach the students a great deal about error propagation and the need for good experimental data2, especially when differences are to be taken (as in calculating a). Careful students achieve remarkably good values for their molecular weight with this modified Dumas bulb. Their values of R, a, P,, and Toshow increasing spread from published values as expected from the nature of the error propagation. We leave it to the student and teacher to interpret the trends in the experimental data and the sources of the experimental uncertainties. Appendix
Table I. The following data were obtained by introducing varying volumes of unknown into a Dumas bulb a t the beginning of the experiment. Note that the unknowns have three differentvapor pressures. Final weight of bulb and u n h (g, IO.WOd g) Volume of liquid unknown introduced (ml)
1JJ 1,l cis 1,2 dichlorotriohlorodichloroethylene ethane ethane (urn = 327 mm) (us = 144 mm) (us = 91 mm)
Use these values to decide how much unknown to introduce into your flash and to estimate the sources and likely size of your experimental uncertainty in grams. Table II. The following data were obtained by removing the Dumas bulb from the water bath a t various times after the Schlieren jet disappeared from the tip of the Dumas bulb.
Time (set)
5 30 60 90 120 180 240
Final weight of bulb and u n h w n (Q, +O.OW*g) cis 1,2 1JJ dichlorotrichloroethylene ethane 67.2731 67.5890 67.2720 67.5870 67.2715 67.5863 67.2712 67.5859 67.2703 67.5854 67.2689 67.5850 67.2534 67.5837
Use these data to decide how long you should leave your
Dumas bulb in the boiling water and to estimate the sources and likely size of your experimental uncertainty in g. Table III. The following data were obtained by weighing a cazefully dried Dumas bulb a t various times after it had been removed from the water bath and cooled under the faucet. Use these data to decide how long you should wait before weighing your Dumas bulb and to estimate the sources and likely size of your experimental uncertainties. Table IV.
cis 1,2 dichloroethylene 66.1023 66.5515 66.5525 66.5528 66.5525 66.5393 66.5120
Time (min) 2 5 10 15 30 120 210
1,lJ trichloroethane 67.72'31 68.4742 68.4745 68.4747 68.4749 68.4748 68.4571
The following data were given t o student,^ to help them identify their compounds.
Substanre acetone ethyl alcohol ethyl acetate 3 pentanone n-hutanol n-butyl chloride 2 methoryethanol t-amyl dclcahal 1,l dichloroethnne cyclohexane cyclopenta~~one 1,1,1 triehloruethrtne 1,2 dichloroethsne chloroform cis 1,2 dirhloroethylene carbon tetrachloride 1,1,2-trichloroethane trichloroethylene 1,3,5 trimethylbenzene ethyl benzoate benzene
Refractive index 1.35886'g.4 1.362421e.' 1.3721618.g 1.39391'.6 1.39931 1.4015 1.402430 1.4052 1.41655 1 .42900'5 1.4366 1.4376521." 1.44432 1.4464318
Molecular weight 58.08 46.07 88.10 86.13 74.12 92.57 76.09 88.15 98.97 84.16 84.11 133.42 98.97 119.39 96.95 153.84 133.42 131.40 120.19 150.17 78.11
-Vapor 20°C
163.3 85.7 18.73 58.9 2.0 73.6
pressure ( 2~5T
205.6 108 4 24.i 82.8 2 8 less than 1 mm !M 0
m m ) ~ 30PC
254.1 134.3 31.6 93 8 3.7 116.1
Volume 44, Number 7, July 1967
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