108 OUTLINES OF PHYSICAL CHEMISTRY
of the solution less the volume of the solvent. This is expressed by the equation :
v m = M "\ a % — ^ cubic centimetres. . (1)
d o
where v m is the molecular volume of the substance, m its molecular weight, aq the weight of the solvent in grams, d the density of the solution, and 3 the density of the solvent (d and 3 being taken at 15° and with respect to water at its maximum density).
By means of this formula, it has been possible to show that the molecular volume in solution is an additive pro- perty. Indeed, the molecular volume of many salts remains unchanged by the substitution of one metal for another, as, for instance, sodium for lithium or hydrogen, or the molecular volume is altered by a constant amount, for instance, by the substitution of potassium for sodium.
But this formula very often leads to results which are too small or even negative.
From the following considerations, L Traube has been able to correct the formula. If we determine the molecular yolume of a non-associated substance in the homogeneous or pure state :
— M
and the molecular volume of the same substance in aqueous
solution :
_ M + qg _aq Vm d T'
we quite generally find
^m = v m — 18-5 c.c. . . . (2)
It is probably not the volume of the dissolved substance which diminishes by 18'5 c.c, but by the formation of new attractions between heterogeneous molecules, the total volume, and more particularly the volume of the solvent
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