Page:Outlines of Physical Chemistry - 1899.djvu/229

 depression of freezing-point can be found from the

equations :

rise in boiling-point ___

, depression of freezing-point __

an ^ _c

where m is the true molecular weight of the dissolved substance. If, on the contrary, the pressure is i times too large, as is the case with electrolytes, then the rise in boiling-point and the depression of freezing-point become equally exceptional and lead to results for the molecular weight which are i times too small.

The value found for m being evidently inversely pro- portional to the number of particles of matter in solution, we are again led to the equation

l s * 1 as 1 + (n — 1) X

N v '

and thus we have new methods of determining x — by means of the boiling-point and the freezing-point.

As the value of x is independent of the method by

tainingw gram-molecules of substance dissolved in g grams of solvent, have been given.

We know that *—/- = m* and that A = b'- (A being the rise

in the boiling-point).

And Arrh6nius has shown that —

p = /&S=-BTS. Now, substituting the values of - of the two first equations in this last, we get

P = J s-J- B T S = ", B TS. /M B ;

N.B. — p denotes the osmotic pressure ; m is the molecular weight (gaseous) of the solvent; t is the boiling-point of the solvent in absolute degrees.

For cryoecopic phenomena analogous relations can be established.

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