Page:Outlines of Physical Chemistry - 1899.djvu/130

 110 OUTLINES OF PHYSICAL CHEMISTRY

dealt with: thus the cis and trans isomers or position- isomers of the aromatic series have not always the same molecular volume in solution.

Bemarks. — 1. Equation (4) is, of course, only applicable

f the assumed molecular weight be equal to the real

molecular weight. Thus the determination of the density

of a solution may be used to control the molecular weight

of the dissolved substance.

2. We have seen that generally

v m = \ m — 18*5 c.c.

This relation, however, is not so simple if the homogeneous

liquid substance is made up of polymerised particles.

In this case, if the polymerised particle consists of n

v w 2= 2 atomic volumes +

n

v m = 2 atomic volumes + (25*9-13'5) and, therefore,


 * „ = v. + 12-4 - ?£?

n

8. For the majority of organic substances, liquid and solid, the molecular volume in concentrated solution in has been found to be equal to the sum of the atomic volumes increased by a co-volume, 25*9 c.c. The dissolved substance is, therefore, present as simple molecules, and, there being but a very slight contraction, the co- volume retains its full value.

4. Solid substances, taken as such, on the contrary, give as molecular volume

-^ = 2 atomic volumes H

a n

which proves that solids are generally not made up of simple molecules but of polymerised particles.

1 Ethyl and methyl alcohol are not to be included.

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