38 DIFFUSION AND OSMOTIC PRESSUKE 



holds for osmotic pressures of dilute solutions. This opera- 

 tion is expressed in the following: 



P T T f =P f T , 



in which P T is the osmotic pressure, in millimeters of mer- 

 cury, at required temperature T (absolute), and T f is the 

 absolute freezing-point of the solution. From the equation 



we get: 



T 



p*=p, Yf . 



In the case of weak aqueous solutions, the freezing-point of 

 the solution may be considered, for this calculation, as prac- 

 tically the same as that of the solvent. Thus Tf =273 

 (the freezing-point of pure water), and T becomes 273 -f- , 

 where t is the desired temperature in the Centigrade scale. 

 Now the equation given above becomes: 



P T = P, (l + gig )=P, (1 + 0.00367*) . 



This is sufficiently accurate for dilute aqueous solutions. 



The freezing-point method is the simplest and most satis- 

 factory method for general use. 



2. The boiling-point method: The boiling-point of a solu- 

 tion is always higher than that of the pure solvent, and its 

 elevation is proportional to the osmotic pressure at that tem- 

 perature. The relation between the two quantities for 

 aqueous solutions is expressed as follows: 



P b = 43320 A,, 1 



wherein P b is the osmotic pressure in millimeters of mercury 

 at the boiling-point of the solution, and A 6 is the elevation 

 of the boiling-point. The determination of the boiling- 

 point of the solution and of distilled water is best made 



i NERNST-PALMER, Theoretical Chemistry (London, 1895), p. 129. The pressure is 

 again reduced to millimeters of mercury. 



