A MOLECULAR-WEIGHT DETERMINATION for FRESHMEN B. G. CARSON Carson-Newman College, Tefferson City, Tennessee
ONVINCED by past experiences that most students leave general chemistry with rather vague ideas about partial pressures, the author of this paper decided to adopt a modification of the steam distillation process as a freshmen laboratory experiment. Preliminary experiments showed that the modified process could easily be carried out with a fair degree of precision, and that the quantitative results could be used by students with little training to calculate the molecular weight of the immiscible organic liquid being distilled. Students apparently have little trouble in applying the law of partial pressures to mixtures of ordinary gases. When it comes to applying this same law to the distillation of liquids most of them lose their way. The statement that the partial pressure of each liquid is proportional to the number of molecules of that liquid present i n the w@or state is misapplied, usually because the student overlooks the fact that the statement refers to the vapors and not to the liquids. Following a discussiou of partial pressures the students are given mimeographed directions for determining the molecular weight of organic liquids immiscible with water. The setup is simply that used for ordinary downward distillation. Graduated cylinders ordinarily used to receive the distillate are replaced by 100-ml. volumetric flasks. Experience has shown that many of the organic liquids do not separate readily from water, and the degree of precision with which one can read a volume with a 100-ml. graduated cylinder leaves much to be desired. In this laboratory the two weighings are done by an assistant using an analytical balance, but there is no apparent reason why a good Harvard trip balance could not be used. Very good results have been obtained using carbon tetrachloride, bromobenzene, n-butyl bromide, aniline, and nitrobenzene. Other liquids immiscible with water should serve equally well, provided the liquid has a density considerably more or considerably less than that of water. Many freshmen obtain molecular weights within a range of two or three per cent. error. The following directions are given to students.
D&:
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A-Weight of flask and distillate.. B-Weightof flask C-Weight of distillate (A - B). ........................... D-Barometer reading.. ................................... E-Boiling paint of mixture.. .............................. F-Vapor pressure of water at temperature E.. ................ &Vapor p r e s m of organic liquid at E . . ...(D - F). H-Temperature of collected distillate.. ..................... &-Density of water at temperature H . . J-Density of organic liquid at H.. ......................... K--Grams of water collected (calculate). L-Grams of organic liquid collected (C - K). M-Number mols water collected (K i 18). N-Number mols organic liquid collected (calculate). .......... O--Molecular weight of organic liquid (L i N ) . ............. P-Percentage error ( m r X 100 i true mol. wt.). ...........
....... ..................... ..................... ............... .................
Calculation of K: Let X = 100 - X = J(100 - X) = IX = J(100 - X) = IX Solve for X K = Calculation of N: F i M =
+
MOLECULAR WEIGHT BY STEAM DISTILLATION Principles inuoloed.-(a) Law of partial pressures. (b) Relations between mass, density, volume.
[email protected] distilling flask, 100-degree thermometer, condenser and tubing, 2 ring stands. 1 ring, condenser clamp,
water bath. small clamp, 100-ml. volumetric M , Mafcrielx-Carbon tetrachloride (chloroform, bromobenzene, aniline, n-hutyl bromide, or nitrobenzene may also be used). Procedure.-1. Set up a distillation outfit like the one already set up in the laboratory. Roll your corks to soften them and see that there is no leak around the thermometer or between the distilling flask and the condenser. The steam bath may be left off if the liquid to be studied is a high boiling one such as aniline, nitrobenzene, or n-hutyl bromide or bromobenzeue, but if the steam bath is left off care must he taken to avoid superheating of the lower layer. In any case the boiling paint of the mixture will he below the boiling point of pure water. (Why?) 2. Weigh the volumetric flask and record the weight ( B ) . 3. Place in the distilling flask, using a funnel, about 200 ml. carbon tetrachloride and 600 ml. water. 4. Wrap a towel around the upper part of the distilling 5 s k to reduce heat lass and heat the steam bath just enough so that the mixture distils drop by drop. (If the steam bath is not used, heat the flask with a low flame.) Discard the &st 10 ml. that distils and continue the distillation till exactly 100 ml. of the distillate has been collected. Record the temperature at which the mixture boils (E). 5. Weigh the flask and the distillate and record the weight (A). Use the thermometer in the distilling outfit to obtain the temperature of the distillate immediately after weighing it. Record this temperature (H). 6. Record the barometer reading (D). 7. Obtain data for (F) and (I)from a handbmk of chemistry and ask an assistant or the instructor for the density (J).
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no. ml. water no. ml. organic liquid no. g. organic liquid no. g. water C IX
G+N
