2l8 



ELEMENTARY CHEMICAL MICROSCOPY 



and if the solutes are non-volatile, the solution with the smaller 

 vapor pressure will have the greater concentration of molecules 

 and vice versa. " 



A series of tubes must be made in which the strength of the 

 standard solution has been systematically varied in small frac- 

 tions of a gram-molecule per liter. A tube is thus obtained in 

 the series where there is little variation in the lengths of the 

 drops of known and unknown or where there is change in the 

 character of the variation, say from an increase in length to a 

 decrease in length. It is evident that the molecular concentra- 

 tion of the unknown must correspond to that of the known solu- 

 tion at this point. 



Weight of unknown in grams per liter 

 Concentration in gram-molecules found 



This may be made clear by quoting one experiment: Standard 

 used, cane sugar. Unknown, glucose. Solvent, water. 



Molecular weight = 



It is evident therefore that the concentration of the unknown 

 material lies between the concentrations of tubes number 4 and 

 5, that is between 0.13 and 0.14 gram-molecule per liter. Hence, 



192. That is, the 



i i i \ 25.02 25.02 



molecular weight = -* 1 = 179, or -^ 

 0.14 0.13 



molecular weight of the unknown lies between 179 and 192; the 

 average = 185.5. Calculated for glucose, C 6 Hi 2 O 6 = 180. 



It appears from a very large number of experiments that this 

 method is a simple and dependable one, apparently subject to 

 errors no greater than those usually inherent in macroscopic 

 molecular weight determinations. 



