��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.
�� �
