Page:Text-book of Electrochemistry.djvu/70

 V.

��OSMOTIC PRESSURE OF A SOLUTION.

��desired. The agreement is much better if Tammann's results be compared with the older determinations of the freezing point by Budorff and de Coppet, and there is no doubt that fresh and more accurate determinations would lead to a much better result.

Connection between the Osmotic Pressure of a Solution and its Freezing Point and Vapour Pressure. — TMs connection was first shown empirically by De Vries in 1884. Soon after, van't Hoflf deduced from the laws of the osmotic pressure both Baoult's law of the depression of the vapour pressure and his own law of the depression of the freezing point ; and in the manner given by van't Hoff I developed the formula for the rise of boiling point.

It may be here noticed that Baoult, after collecting a very large number of data on the freezing points of solutions, empirically found a connection which he expressed in the following formula : —

dT = 0-63 X n.

According to this formula, 0*63 x n is the depression of the freezing point of a solution which contains n molecules in 100 molecules of solvent. This formula only agrees with the law of van't Hoflf when applied to formic acid, acetic acid, and benzene, for which the law requires the values 0-62, 0-65, and 068. On the other hand, the value for water is 1'05, and Baoult takes this to indicate that some of the water molecules have condensed to complexes 2H2O and 3H2O. In this connection Eykman (tf) carried out an investigation, in which he obtained the following results : —

��Solvent

�<ir(obwryed).

�dT (caleaUted).

�cir(calcuUte(0- lUouU.

�Phenol ....

�Naphthalene. . . p-Toluidine. ..

�Diphenylamine. .

�Naphthylamine. .

�102-5 .?)

�Laurie acid. ..

�Palmitic acid. ..

i

�� �