261 
1910-11.] Molecular Weights of Dissolved Substances. 
temperature equilibrium is attained. To obtain the lowering of vapour 
pressure, the reading is corrected by the value of the zero reading already 
found. The stopper is removed, and the volume of the solution read off. 
From the observed quantities, — weight of solute W, volume of solution 
V, and lowering of vapour pressure L — the molecular weight may be 
calculated from the formula : — 
w X B 
M= 1000 
L. Y. 760 
where B is the barometric height, and K 760 is a constant for the solvent, 
simply deducible from theoretical considerations. 
The values of K 760 for some of the common solvents are as follows : — 
Solvent. 
K r60 . 
Solvent. 
^760- 
Benzene 
1214 
Ether 
1577 
Alcohol 
871*5 
Carbon bisulphide 
526*6 
Water 
202*5 
Ethyl acetate 
1320 
Chloroform 
Acetone 
620*4 
1061 
Ethylene dibromide 
514*6 
The results tabulated below will serve to indicate the order of accuracy 
that may be attained by a beginner. For these determinations I am in- 
debted to Mr Severin Gertken, who was without previous experience of 
molecular weight determination by any method. 
Solvent. 
Solute. 
M. Wt. 
normal. 
M. Wt. 
found. 
Beckmann 
found. 
Benzene . . \ 
Carbon bisulphide j 
Water . . j 
Benzil 
Ethyl Benzoate 
Benzil 
Benzophenone 
Naphthalene 
Boric acid 
Potassium chloride 
210 
150 
210 
182 
128 
62 
74*6 
203 
164*5 
206*5 
195*9-196*3 
127*6 
61*4-62*1 
43*2-44*2 
216-256 
163-172 
217 
182-187 
131-141 
64*6-66*9 
University of Chicago, 
May 1910. 
(Issued separately February 23, 1911.) 
