Page:Outlines of Physical Chemistry - 1899.djvu/227

 as ammonium chloride, phosphorus pentachloride, ammo- nium carbamate, &c, in the gaseous state occupy a volume which is m times greater than that expected, and, conse- quently, under normal volume condition they exert a pressure which is m times too great. In order to explain this we assumed that in passing into the gaseous state the molecule is decomposed into smaller particles.

An analogous assumption can be made to explain the irregularity under discussion. If a dilute aqueous solution develops an osmotic pressure i times too great, 1 it is pro- bable that the solution contains i times more dissolved particles than would be expected from the unitary formula of the substance dissolved.

Thus the exceptions to varCt Hoff's law lead us to the same hypothesis as we have formulated to explain the conductivity of solutions. And it may be at once added that the agreement between these two classes of pheno- mena is not only qualitative, but can also be followed quantitatively.

If we take a salt solution of molecular conductivity /*, and which has an osmotic pressure i times too great, what will be the value of the fractional number x, expressing the degree of dissociation ?

The first reply to this question is furnished by electro- chemistry, because from the determined molecular conduc- tivity we can calculate x from the equation :



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��From the osmotic point of view it is not any more difficult to ascertain the value of x. We know to what number of ions the chemical composition of the dissolved substance corresponds, and for the present we may assume (as Arrh&nius does) that the dissociation consists in a liberation of the ions. Now, if the dissociation were zero,

��1 I.e. i times the pressure calculated from van't Hoff's law.

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