ISOTONIC SOLUTIONS 193 



of a substance is defined as the number of gram-molecular weights M 

 contained in 1000 grams of water. 



As a second step another aspect of the general gas law must be justi- 

 fied, namely : the pressure developed by a given mass of gas, at constant 

 volume, is proportional to the absolute temperature. The restatement 

 of the law as applied to dilute solutions would read : the osmotic pressure 

 developed by a given concentration is proportional to the absolute 

 temperature. Table V-4 shows that a sucrose concentration of 0.4 M 



TABLE V-4 

 Osmotic Pressure of Sucrose 



Concentrations in gram-molecular weights per 1000 grams water. 

 Sucrose C12H22O11. Temperatures in degrees Centigrade. 

 Osmotic pressure in atmospheres, standard conditions. 

 C = M/1000, where M = 342.2. 



By Courtesy of Morse, Holland, Myers, Cash, and Zinn, Am. Chem., J., 48, 29 {1912). 



at 10° C can develop an osmotic pressure of 9.87 atmospheres while the 

 same concentration at 20° C should, according to the general gas law, 

 develop a pressure of 10.21 atmospheres. The data show that this 

 sucrose solution can develop an osmotic pressure of 10.22 atmosphere. 

 The conclusion is that, at any given temperature, all equal concentra- 

 tions of dilute solutions of non-electrolytes have the same osmotic pres- 

 sure. Hence, the molecular weight of a non-electrolyte being known, 

 the general gas law can be used to calculate the amount of a substance 

 that must be dissolved in 1000 grams of solvent to obtain any desired 

 osmotic pressure. 



Isotonic Solutions 



Solutions developing the same osmotic pressure are isosmotic or iso- 

 tonic. Hypotonic solutions are solutions that have a lower osmotic 

 pressure, and hypertonic solutions possess a greater osmotic pressure, 



