LAWS OF SOLUTION 



obey the same laws as .those in gaseous form, we must employ a semi- 

 permeable membrane which is rigid enough to withstand the pressure 

 and which forms part of the walls of a closed vessel connected with a 

 manometer. If we place in such an osmometer a solution containing the 

 molecular weight in grams of some substance dissolved in one liter of 

 solvent, a so-called gram-molecular solution, it is obvious that, if the 

 gas laws are to apply, the osmotic pressure should equal that of 22.4 

 liters of a gas compressed to the volume of one liter; in other words, 

 it should equal 22.4 x 760 = 17.024 mm. Hg. Although there are very 

 considerable technical difficulties in making a semipermeable membrane 

 that is strong enough to withstand such a pressure, yet this has been accom- 



w 



Fig. 1. Diagram of osmometer. The cylindrical vessel (O), with a bottom of unglazed 

 clay, the pores of which are filled with a precipitate of copper ferrocyanide to form a semi- 

 permeable membrane, is suspended in an outer vessel, and is closed above by a tightly fitting 

 stopper pierced by a tube leading to a manometer (M). O contains a strong solution of cane 

 sugar, and W contains water. The water molecules tend to pass through the semipermeable 

 membrane into the cane sugar solution, and since the cane sugar molecules can not pass in 

 the opposite direction, the pressure in O rises and is recorded in M. This equals the osmotic 

 pressure. 



plished, and the fundamental principle has therefore been firmly estab- 

 lished that substances in solution obey the same laws as gases. 



Further proof that the gas laws apply to solutions has been secured by 

 showing that the osmotic pressure (of a dilute solution) is directly pro- 

 portional to the concentration of the dissolved substance (the solute) 

 and to the absolute temperature. It also obeys the law of partial pres- 

 sures, which states that the total pressure exerted by a mixture (of gases 

 or dissolved molecules) is the sum of the pressures which each constit- 

 uent of the mixture would exert were it alone present in the space 

 occupied by the mixture. 



