6 THE PROPERTIES OF SOLUTIONS. [CH. I. 



Consequently the osmotic pressure of a solution is most con- 

 veniently estimated by a determination of its freezing 

 point. 



The depression of the freezing point (A) for a given 

 concentration of a substance varies with the solvent 

 employed, the relationship for non-electrolytes being 



- x M = C, where w is the weight of a substance of mole- 

 w 



cular weight M dissolved in 100 grams of the solvent and 

 C is the "coefficient of depression" for the particular 

 solvent. If 5 grms. of solvent are taken instead of 100, it 

 follows that since A is proportional to the concentration 



s x - x M = C 



IOO W 



The value of C for water is 18-6 C. : for acetic acid it is 

 39 C. 



Van 't Hoff ha^s shewn that the value of C can be 



2 T 2 



calculated from the formula C = - - . where T is the 



loo L 



absolute temperature of the freezing point, and L is the 

 latent heat of fusion of the solvent. 



Thus for water T = 273 

 L = 80 







So C = 2 X = 18-6. 



IOO X 80 



With non-electrolytes, therefore, the gramme-molecule 

 in 1,000 gm. of water causes a depression (A) of 1-86 C. 



So that -- = molecular concentration. 

 1-86 



For electrolytes : = concentrations of (ions + 



i '86 



molecules), so that if a substance be ionised to the extent 

 of i per cent, the molecular concentration is 



A x loo 

 1-86 x (21 + loo- i) ' 



