3o 



PHYSICAL CHEMISTRY 



Fig. 6. Graph indicating the method of integrating the formula PV=RT 

 (from MRR). 



ume in liters in which i mol solute is contained, R = 0.0821, and 

 T is the absolute temperature, in which zero is — 273.090 on the 

 centigrade scale (Fig. 6). Since the volume is the reciprocal of 



p 



the concentration, the formula becomes — = 0.0821 T, where 



c 



c is the molecular (molar) concentration. If the concentration 



is 1 and the temperature = o° C = 273 abs., p = .0821 X 273 



= 22.4. In other words, the osmotic pressure of a molar solution 



is 22.4 atmospheres at o°. From this a method was developed 



of determining the extent of dissociation of an electrolyte from 



its osmotic pressure. If 1 mol KG is dissolved in so large a 



volume of water that it is completely dissociated it will exert 



double the pressure of 1 mol dextrose dissolved in the same 



volume, because the number of ions is double the number of 



molecules, and the formula becomes pv = 2RT. If the salt is 



partly dissociated a number somewhere between 1 and 2 must 



be used (say 1.5), then 50 per cent of the salt is dissociated. 



The most accurate determinations of osmotic pressure have 



been done on sugars by H. N. Morse (1914) and Berkeley and 



Hartley (1904). Morse made some determinations on elec- 



