Page:Outlines of Physical Chemistry - 1899.djvu/148

 130 OUTLINES OF PHYSICAL CHEMISTRY

heat of vaporisation of one gram of the liquid. The whole system is then raised to the temperature (t + A)°, the boiling point of the

solution, and the 2 grams of vapour are allowed to condense in the

n

solution : the heat of vaporisation formerly dissipated at t° is thus regained at the slightly higher temperature (t + A)°. The whole system is finally cooled to t° and thus brought back to its initial state.

This cycle may be carried out so that it is reversible, and leads

to the equation :

bt _ A

��n bt* n

��Therefore, A =

��But A = b'- , n

��consequently. b' = —.

In the thermodynamic^ equation b represents a quantity of heat, and we have already seen that its value is 2 small calories ; therefore,

we write

r _ 2t*

��e' =

��I

��Remark, — We have arranged in this equation that a' refers to one molecule of dissolved substance in one gram of solvent.

In the chapter on Boiling Points we made use of the constant b which referred to one molecule of substance in 100 grams of solvent,

0*02 t* and, consequently, is 100 times smaller than e\ that is, e =

��I

��C. Osmotic Pressure and Cryoscopy

��Here also we shall use a very dilute solution containing n molecules of substance dissolved in g grams of solvent. In this case t denotes the temperature (absolute) at which the solvent freezes The solution commences to freeze at the temperature (t - A). The cryoscopic constant c' refers to a solution of one molecule of

substance in one gram of solvent, and c' = A.?.

n

Thermodynamical calculation of the constant c' (vanH EoffY

In an osmotic cell, fitted as described under calculation of e', we put

a large quantity of the solution. At the temperature t° a quantity

�� �