CH. I.] FREEZING POINT. 7 



The quantitative relationship between osmotic pres- 

 sure and A for aqueous solutions can be readily calculated 

 as follows. 



The gramme-molecule in 22-4 litres gives an osmotic 

 pressure of 760 mm. of mercury at o C. 



The gramme-molecule in i litre gives a A of i -86 C. 



So the gramme-molecule in 22-4 litres gives a A of 



1-86 (0 ^ 



= 0-083 C. 



22-4 



So a A of 0-083 C. corresponds to an osmotic pressure 

 of 760 mm. 



So a A of o-oo i C. corresponds to an osmotic pressure 

 of 9-1 mm. 



Thus a 5 per cent, solution of glucose (Mol.wt. = 180) 



has a A of 1-86 x -^- = 0-517 C., 



1 80 



and an osmotic pressure of 51-7 x 9-1 = 470 mm. Hg. 



The A of Blood is about 0-55 C., corresponding to an 

 osmotic pressure of about 5oomm.Hg. 



Owing to the relatively small number of particles in a 

 given volume of a colloidal solution, it follows that the 

 freezing point of such solutions is only very slightly lower 

 than that of distilled water. Since it is very difficult to 

 remove the last traces of electrolytes by dialysis, it is not 

 easy to obtain reliable figures for the osmotic pressure of 

 the colloids. Sorensen has recently investigated the problem 

 and has been successful in overcoming the technical diffi- 

 culties. He states that the osmotic pressure of crystalline 

 egg-albumin indicates a molecular weight of 34,000. 



5. The determination of the freezing point by Beckmann's 

 method. (Cryoscopy.) 



Take the freezing point of (a) distilled water; (b) M/5 NaCl 

 (1-16 per cent.) ; (c) M/5 glucose (3-6 per cent.). 



