Page:Outlines of Physical Chemistry - 1899.djvu/140

 ��OUTLINES OF PHYSICAL CHEMISTRY

��(1) We can dissolve a substance of well-known molecular weight in it, and determine a series of correlative values of

t and p, then apply the equation - m = e ;

P or (2) We can derive the value of e by making use of the

relationship established by van't Hoff, which is expressed

by the equation (to be gone into more fully later) :

where I is the latent heat of vaporisation of one gram of the solvent and t is its boiling point on the absolute scale. The following are some of the values found for e, the boiling point being also given :

Boiling point

�� � �(Centigrade)

�E

�Ethyl ether 35°

�Acetone

� � �56°

�Methyl alcohol

� � �. 66°

�Ethyl alcohol

� � �. 78°

�Water.

� � �100°

�Acetic acid.

� � �. 118°

�Phenol.

� � �. 183°

�Carbon disulphide

� � �. 46°

�Carbon tetrachloride

� � �. 76°

�Benzene

� � �80*3°

��Practical Application. — When once we are in posses- sion of this table, we can follow the reverse process, and by means of the rise in the boiling point determine the molecular weight of a dissolved substance.

From the knowledge simply of p and t of a single solution, the value of m can be calculated from the equa- tion m = e^. But, in practice, it is necessary to examine

V

a series of solutions of different concentrations, and from the observed correlative values of p and t, m is calculated, and a graphic curve (something like the one shown in fig. 80) of the values obtained is drawn.

�� �