Page:Outlines of Physical Chemistry - 1899.djvu/129

 (water), is diminished by that amount. Be that as it may, the contraction produced affects the value of the molecular volume as calculated by equation (1).

We have already seen that the molecular volume of a homogeneous substance is equal to the sum of the atomic volumes increased by the co- volume 25.9 c.c. In aqueous solution the same rule holds good, but the co-volume is 18.5 units smaller, and so we have the following equations for the molecular volume of a substance in the pure state and in solution:

v m = -=- = S atomic volumes 4- 25.9 c.c. . (8)

v m = }±+M _ °3 = 2 atomic volumes + (259-18.5) c.c. (4)

For dilute (1 to 3 per cent.) aqueous solutions at temperatures near 15°, we can calculate the sum of the atomic volumes from Traube's constants given on page 68. Every ring formation of six carbon atoms produces a further contraction of 8*1 c.c. Double or triple linkage between two carbon atoms seems to be without influence on the resulting volume (?).

This theory has been applied to a large number of organic liquid substances and even to some solids: acids, alcohols, ethers, ketones, amides, amines, phenols, &c,, and the difference between the calculated and the experimental v m has only rarely been as much as two or three units.

This is due to the fact that irregularities due to polymerised or associated particles do not present themselves. In aqueous solution the molecules seem to be as simple as in the gaseous condition. Even for substances which associate very readily, such as methyl alcohol, acetic acid, and glycerine, the molecular volumes in dilute solution are normal, corresponding to the values calculated by equation (4). Complications, however, arise with the influence of structural peculiarities which cannot yet be satisfactorily

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