MEASUREMENT AND CALCULATION 41 



sulphuric-acid tube gives the required data. The amount of 

 vapor removed from the two bulbs respectively is proportional 

 to the vapor tensions of their contents. Thus if w denote the 

 loss in weight in the second bulb and w' the gain in weight 

 of the sulphuric acid, 



w IT it' 



w' TT 



Therefore the equation given above may be written: 



w 



This method is difficult of operation and not very satis- 

 factory. The whole apparatus must be surrounded by a 

 jacket to keep all the parts at the same temperature; it is 

 not necessary that the temperature be absolutely constant, 

 however. The only advantage in this method over those 

 previously described is that by this means the osmotic 

 pressure can be determined for the temperature at which 

 the solution is used, thus avoiding the correction for tem- 

 perature. 



II. CALCULATION OF OSMOTIC PRESSURE 



a) When the pressure is produced by a non-electrolyte. 

 All solutions of non-electrolytes which contain the same 

 number of molecules per unit volume of solution give the 

 same osmotic pressure. From measurements made by Pfeffer 

 we know that the osmotic pressure of a solution of sugar 

 containing a gram-molecule per liter is the same as the gas 

 pressure of a gram-molecule of gas occupying a liter volume. 

 This pressure is 22.3 atmospheres, or 16,948 mm. of mercury, 

 at C., or 273 absolute. Thus, if we know the molecular 

 weight of the solute and the number of grams per liter of 

 solution, the calculation, on the principle that pressure varies 

 as concentration, is simple enough. The correction for tem- 

 perature is carried out by the principle of Gay-Lussac. 



