LIQUID SOLUTIONS 21 



two different molecules. Now, if these two masses are placed 

 in equal volumes of solution, both solutes ought to show the 

 same diffusion tension. This, indeed, is found to be true, 

 and the same principle has been shown to be true, as far 

 as experiment has gone, for solutions of all substances 

 which do not conduct electricity (non-electrolytes). Solu- 

 tions of non-electrolytes which contain the same number 

 of molecules per unit volume have the same diffusion tension 

 (at the same temperature) and are physically similar. Solu- 

 tions which conduct electricity exhibit this principle only in 

 a general way. Their departures from it and the reasons 

 therefor will be discussed in the next chapter. 



The number of grams of a substance represented by its 

 molecular weight is called a gram-molecule. Gram-molecules 

 of all substances contain, then, the same number of molecules. 

 If a gram-molecule of some substance be put into solution, and 

 then this be diluted to one liter, there results a solution which 

 can reasonably be used as a standard. Such a solution is often 

 termed a molecular solution. Thus, a molecular solution of 

 potassium nitrate (mol. wt. 101) is 101 grams of the salt in 

 a liter of solution. It is as though the substance had been 

 vaporized and the resulting gas occupied a volume of one 

 liter. 



But the analytical chemist has found it convenient to use 

 another solution as a standard. He dissolves, to form a liter 

 of solution, as many grams of the substance in question as 

 will react chemically with a gram -molecule of a monovalent 

 compound. This amount of substance is termed a gram- 

 equivalent. A gram-equivalent of sulphuric acid (H 2 SO 4 ) 

 will just neutralize a gram-equivalent of potassium hydroxide 

 (KOH) or will just decompose a gram-equivalent of sodium 

 chlorid (NaCl) ; but it takes two gram-equivalents of either 

 of the last-mentioned compounds to react completely with a 

 gram-molecule of sulphuric acid. It follows that "gram- 



