Chapter III 

 PROPERTIES OF SOLUTIONS 



Introduction : — Studies on the structure of water have tended to single 

 out the water molecule as being most influential in molding the molecular 

 architecture of aqueous solutions ; where solutes have been considered, the 

 primary emphasis has been on the effects of solute molecules upon the ar- 

 rangement and properties of water molecules. 



The Interrelation of the Properties of Solutions : — Studies on solu- 

 tions have tended to center about the effects of solute molecules upon such 

 properties of the solution as osmotic pressure, thermal properties, vapor 

 pressure, and the like. Most of the calculations have involved an equation 

 of state, that is, an equation expressing the relations between pressure, 

 volume, concentration, temperature, and similar physical characteristics. 

 Starting from the concept of an ideal solution, they have attempted to recon- 

 cile deviations from ideality with known or predicted properties of the 

 solute and solvent. 



The first comprehensive theory of dilute solutions was founded by 

 van't Hoff (1887, 1888). Acquainted by the Dutch botanist de Vries 

 with the osmotic experiments of Pfeffer, he formulated an equation re- 

 lating osmotic pressure with the concentration of a solution. The expres- 

 sion is 



P„V = nRT (i) 



where P^ = osmotic pressure of the solution, 

 V = volume of the solution, 

 n = number of mols of solute, 

 R = the molar gas constant, and 

 T = absolute temperature. 



Proving that for dilute solutions the value of the constant R was identi- 

 cal with that of the gas equation, van't Hoff arrived at the important 

 generalization that the osmotic pressure of a dilute solution is equal to the 

 pressure that the molecules of the solute would exercise if they existed in 

 a gaseous state in the volume occupied by the solution. As pointed out by 

 FiNDLAY (1919), the unique value of this finding lies in the fact that the 

 concept of osmotic pressure and the use of semi-permeable membranes 

 allows the properties of solutions to be treated like those of gases, particu- 

 larly the quantitative relation between colligative properties and concen- 

 tration. 



Though his law has been misused, van't Hoff realized that it applied 

 only to very dilute solutions. This is apparent from the expression given 

 because, in it, osmotic pressure varies as the ratio of mols of solute per 

 volume of solution, whereas in an exact law it should depend upon the 

 ratio of solute to solvent molecules and the forces between them. The 

 above law provides no correction for the effect of the solute molecules upon 

 the properties of the solvent or vice versa. 



