Page:Outlines of Physical Chemistry - 1899.djvu/143

 Application. — When the value of c has been deter- mined, cryoscopic experiments can be made in order to find the molecular weight of a substance. The determina- tion of the freezing point of a single solution of the substance, and calculation from the equation :

should theoretically be sufficient. But, in practice, it is absolutely necessary to calculate m from a series of obser- vations on solutions of different concentrations and make a graphic curve of the values obtained, marking off the depressions on the abscissae-axis and the values of m on the ordinate. The curve generally shows that the most satisfactory results are those obtained from very dilute solutions. Concentrated solutions may contain the substance in a more or less associated state and thus lead to much too high values for the molecular weight.

The most abnormal values are obtained for substances containing one or more hydroxyl groups (alcohols, organic acids, phenols, oximes, and also amides) in certain solvents possessing an associating power such as benzene, naphthal- ene, and nitro-benzene.

Important Bemark. — Those substances which we have already noticed as exceptions at the end of the last two chapters behave here also in an abnormal manner : Elec- trolytes, in aqueous solution, always yield much too low results for the molecular weight by the cryoscopic method. It is to be noted, however, that amongst the electrolytes we must not class certain acids and bases whose aqueous solutions are poor conductors of an electric current and which give a normal depression of the freezing "point. Sulphurous acid, hydrogen sulphide, and ammonia belong to this category.

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