Page:Outlines of Physical Chemistry - 1899.djvu/141

 VAPOUR TENSION AND BOILING POINT

��The curve m m' generally approaches to a line parallel with the abscissae. But in certain cases (such as phenan- threne in benzene solution) the values found from the more concentrated solutions are too high, corresponding to an association of the mole- cules. In the more dilute solutions, this anomaly is not so marked, and from the course of the curve we can see what the (true) molecular weight would be at infinite dilution.

This irregularity occurs most often when we make use of the last three solvents mentioned in the list, and more especially with benzene. The first seven solvents on the list, on the contrary, have a dissociating action, and it is only in very rare cases that polymerised molecules can remain dissolved in them.

Bemark. — Electrolytes, which form the exception to van't HofTs osmotic pressure law, behave, in respect to the raising of the boiling point, also in an abnormal manner. In aqueous solution the mo- lecular weight found for them by this method is only a

fraction of the real value. Thus, for sodium chloride of molecular weight 58*5, the boiling-point method with a dilute solution of the salt gives only about 29*2.

As before, this difficulty can only be explained after we have learned some more about salt solutions.

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