CH. I.] OSMOTIC PRESSURE. 5 



If w grams of a substance be dissolved in V cc. of solvent at t C. and 

 the osmotic pressure produced be P. mm. of mercury, the volume at o C. and 

 at 760 mm. mercury will be 



V x 273 x P 

 (2 73 T~t) x 760 = V 



Now V contains w grams of substance, so 22400 cc. contains 

 22400 x w 



Since this weight of substance produces a pressure of 760 mm. at o C. when 

 in a volume of 22-4 litres it follows that it is the molecular weight of the 

 substance. 



Instead of the formula V.P. = R.T., we must use the following for electro- 

 lytes, 



V.P. = 2 * + ( I0 " *) R.T., where i is the percentage of the substance 

 100 



ionised. 



3. Chemical garden. To a dilute aqueous solution of potassium 

 ferrocyanide add a particle of solid ferric chloride. A film of 

 Prussian blue is formed round the solid. This membrane is semi- 

 permeable, and allows water to pass in to dissolve the ferric chloride. 

 The osmotic pressure of this solution being greater than that of the 

 solution outside, water passes into the cell, which expands, and may 

 assume remarkable forms. 



4. A drop of a fairly strong solution of potassium ferrocyanide 

 is added to a dilute solution of copper sulphate. A semipermeable 

 cell of copper ferrocyanide is thus formed around the drop. The 

 osmotic pressure of the potassium ferrocyanide being greater than 

 that of the copper sulphate, pure water passes from the sulphate 

 into the cell. This results in a concentration of copper sulphate 

 immediately around the cell, and blue striae can be seen descending 

 owing to the greater density of the strong copper sulphate solution 

 thus formed. 



D. Freezing Point. 



The freezing point of a solution of a substance is 

 always lower than that of the solvent. The depression of 

 the freezing point (A) depends on the number of particles 

 in a given volume of the solution. We have already seen 

 that the osmotic pressure of a solution also varies with 

 the number of particles in a given volume of the solution. 

 It therefore follows that A varies with the osmotic pressure. 



