LAWS OF ELECTROLYTIC DISSOCIATION 13 



related to the high degree of dependance of the dissociation constant 

 upon the temperature is the relatively high heat of dissociation of 

 water. 



The dissociation constant of water kw at 22°C. amounts to 1 X 

 10~", i.e., one ten-millionth squared. From this it follows that in 

 pure water the H+- and OH~-ions are each present in a concentra- 

 tion of 1 X 10~^, or, one ten-millionth gram-ions per liter (one 

 ten-millionth normal in respect to each ion) . If this figure seems to 

 be extremely small, it is, nevertheless, a sharply defined value. If 

 it be recalled that one gram-molecule of any substance contains 

 6.2 X 10-^ molecules (Loschmidt), then in pure water there must be 

 present 6.2 X lO'^* X 10"^ = 6.2 X lO^^ of H+- also of OH --ions 

 per liter, or 6.2 X 10^*^ = 62 billion per cubic millimeter. 



One liter of water contains 1000/18 = 55.56 mols of H2O, or 55.56 

 X 6.2 X 10^3 = 344 X lO^^ molecules of H2O. The same liter on 

 the other hand contains, as seen above, 6.2 X 10^® H+-ions. 

 Therefore, out of every 555 million H2O molecules one is dissociated. 



4. The properties of water and of its ions. The definitions of acids, 



bases and ampholytes 



Water occupies a most unique position among liquid compounds. 

 Leaving aside some of its peculiarities, such as its maximal density 

 at 4°, its expansion on freezing, its very great surface tension, etc., 

 only those of its properties will be dealt with here which affect its 

 behavior as a solvent for electrolytes. These special properties are 

 best explained on the basis of its exceptionally large dielectric con- 

 stant. The significance of this important factor may be conveyed 

 as follows: Two electrically and oppositely charged particles of 

 which one carries a positive charge ei and the other a negative 

 charge e2 and separated by a distance r are attracted towards each 



other with the force equal to , when the two are in a vacuum 



(or with practically the same force in a gaseous medium). When, 

 however, the same two particles are found in a solid or liquid 

 medium, the force of attraction operating between them becomes 



smaller, and its formulation is now — =^. In this expression D 



represents the dielectric constant of the medium. The greater the 

 value of D of a medium the smaller is the electrostatic attraction 



