A Modified Rast Method for Molecular Weights The use of the classical Rast method for the determination of molecular weights in the undergraduate laboratory has been avoided because of the problem associated with the preparation of a solution of the compound in camphor. Heating the two usudly causes apertial sublimation and loss of camphor unless a sealed tuhe is used. This difficulty of preparation can be avoided by dissolving weighed samples of camphor and compound in methylene chloride and evaporating the methylene chloride. The procedure devised and given below gave reasonable results with General Organic Chemistry students using the conventional capillary melting point apparatus. The distribution of molecular weights found is shown in the table. Procedure. The weight of a small clean dry test tuhe fitted with a. cork is determined accurately (four decimal places). Always handle the test tube with Kimwipes or filter paper instead of your bare hands to prevent fluctuation in weights. Approximately 3&50 mg of the unknown is placed in the tuhe and the tuhe is weighed agein accurately. Then about 0.5 g of D-camphoris added, and the tubeis weighed again accurately. To the contents of the tube 3 ml of methylene chloride is added and the solution is stirred with a glass rod until all the solid has dissolved. Be sure the solution is homogeneous. Pour out 0.5-1 ml of the solution on a clean watch glass and allow the solution to evaporate. After standing for 15 min the solid formed on the watch glass is scraped together and mixed well until a fine powder is t with this solid to a height of 3 mm and determine the melting point; the melting ohtarned. Fill a meltine ~ o i ntube &nt r x n m will IRTPP. The temoerature at the I& cnrstal of soliddisa~oearsis taken asthe meltin~ooint of -.~.~. ..~~~-. --- he ~. -~~~ .. e~ ~ ~ which the mixture. D-Camphor after similar treatment melts at 178°C. It is reported that the mold depression constant K for n-camphor is 39.7 for solute concentrations above 0.2 M. More dilute solutions cause an increase in oonstant to about 50. The unknowns used in this experiment have molecular weights which at the value of sample used will give greater than 0.2 M solutions.
-.
.~~
..
~~
Distribution of Molecular Weights Found
Comoound
Molecular weizht
&lo
Experimental molecular weight deviations in percentages +20 +30 &40 450
>50
Volume 49, Number 6, June 7972
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