| Then we have, K = | C | × M. |
| P |
K is a quantity varying with the nature of the solvent but with the same solvent remaining sensibly constant for numerous groups of compounds.
The value of
| A ( | C | ) |
| P |
can be determined by experiment. The molecular weight can therefore be calculated from the formula
| M = | K |
| A |
With organic compounds in water the value of K is almost constant.
Brown and Morris[117] report results of their work in extending Raoult’s investigations of the molecular weight of the carbohydrates. The process is carried on as follows:
A solution of the carbohydrate is prepared containing a known weight of the substance in 100 cubic centimeters of water. About 120 cubic centimeters of the solution are introduced into a thin beaker of about 400 capacity. This beaker is closed with a stopper with three holes. Through one of these a glass rod for stirring the solution is inserted. The second perforation carries a delicate thermometer graduated to 0°.05. The temperature is read with a telescope. The beaker is placed in a mixture of ice and brine at a temperature from 2° to 3° below the freezing point of the solution. The solution is cooled until its temperature is from 0°.5 to 1° below the point of congelation. Through the third aperture in the stopper a small lump of ice taken from a frozen portion of the same solution, is dropped, causing at once the freezing process to begin. The liquid is briskly stirred and as the congelation goes on the temperature rises and finally becomes constant. The reading is then taken. The depression in the freezing point, controlled by the strength of the solution, should never be more than from 1° to 2°.
The molecular weights may also be determined by the boiling points of their solutions as indicated by the author,[118] Beckmann,[119] Hite, Orndorff and Cameron.[120]