JOURNAL O F CHEMICAL EDUCATION
UNKNOWNS FOR MOLECULAR WEIGHT DETERMINATION H . D. CROCKFORD Venable Chemical Laboratory, University Chapel Hill,North Carolina
of North Carolina,
T ~ determination E of molecular weight by the Victor and the variation in the molecular weight is roughly Meyer method is one of the most firmly established three units. Many more variations are, of course, experiments in chemistry. However, its usefulness is possible. The formula for calculating the molecular somewhat l i i t e d when carried out with the usual weight of a sample follows: laboratory apparatus due to the limited number of sub1.48 X 74.1 X 119.4 X ml. CHCI. + stances available for the determination. But un- Molecular weight = 74.1 X 1.48 X ml. CHCb + 0.71 X 74.1 X 119.4 X ml. ether knowns may be prepared by the following method. 119.4 X 0.71 X ml. ether Various proportions of ethyl ether and chloroform are measured out from suitable burets into small cork13,100 X ml. CHCI: + 6284 X ml. ether stoppered bottles. In this way a large series of samples Molecular weight = 109.7 X ml. CHCL + 84.8 X ml. ether can be secured with "molecular weights" varying from where 1.48 and 0.71 are the densities of chloroform and that of pure ether to that of pure chloroform. The ether and 119.4 and 74.1 their molecular meights. series used in this laboratory is given in Table 1. It The procedure used in this laboratory is to require a will be noted that the total sample prepared is 15 ml. student to secure satisfactory checks with a pure chemical, after which be is then required to secure checks of TABLE 1 the same order of accuracy with an unknown from the CHCb,ml. E t h e ~ , m l . MolenclarWeight above series. The determination is carried out with a Sample No. 1 14 77.9 small glass bulblet as described in Experiment 2 of "A 1 2 2 13 81.6 Laboratory Manual of Elementary Physical Chemis12 85.1 3 3 try," by Mack and France (D. Van Nostrand Co.). 4 4 11 88.5 5 5 10 91.8 The bent-over end of the bulblet is immersed directly 6 6 9 95.0 in the bottle containing the sample. Filling is effected 7 7 8 98.1 by the usual technique. By filling directly from the 8 8 7 101.1 sample bottle possible change in concentration due to differential evaporation is prevented. Experience over a number of years has demonstrated that the results obtained with the mixtures are as accurate as those obtained with the pure substances.
