Page:Outlines of Physical Chemistry - 1899.djvu/142

 ��OUTLINES OF PHYSICAL CHEMISTRY

��100 grams of solvent, then the molecular depression c

will be - m, where m is the molecular weight of the dis-

V solved substance.

The value of c is independent of the nature of the substance dissolved, and varies only with the solvent employed.

For any given solvent, there are two methods of ascer- taining the value of c.

(1) We may determine the depression caused by the solution of a certain weight of a substance of known molecular weight in a certain amount of the solvent, and calculate c from the equation

c = -M

P

or (2) We may find c from varit Hoffs formula : l

C = :.

��in which t is the freezing point (in absolute degrees), and \ is the latent heat of fusion of one gram of the solvent.

The following table gives the values of c (experimental and calculated by varCt Hoff's formula) and of t and A.

�� �c

�C

�T

� �—

�experi-

�calcu-

�abso-

�A

� �mental

�lated

�lute

! Water.

�o

�79*4 calories (small)

�Acetic acid.

�Phenol.

�Formic acid.

� �Ethylene bromide

� �Benzene

�Naphthalene

�Nitro-benzene

i

��1 In his calculations varCt Hoff denotes by p the weight of substance dissolved in (100 — p) grams of solvent or 100 grams of solution. For dilute solutions the difference between this and BaoulVs system is very small.

�� �