148. Molecular Weights of Carbohydrates.—In the examination of carbohydrates the determination of the molecular weights is often of the highest analytical value.
The uncertainty in respect of the true molecular weights of the carbohydrates is gradually disappearing by reason of the insight into the composition of these bodies, which recently discovered physical relations have permitted.
Raoult, many years ago,[116] proposed a method of determining molecular weights which is particularly applicable to carbohydrates soluble in water.
The principle of Raoult’s discovery may be stated as follows: The depression of the freezing point of a liquid, caused by the presence of a dissolved liquid or solid, is proportionate to the absolute amount of substance dissolved and inversely proportionate to its molecular weight.
The following formulas may be used in computing results:
C = observed depression of freezing point:
P = weight of anhydrous substance in 100 grams:
| C | = A = depression produced by one gram substance in 100 grams: |
| P |
K = depression produced by dissolving in 100 cubic centimeters a number of grams of the substance corresponding to its molecular weight:
M = molecular weight: