ACIDITY 303 



concentration of the undissociated molecules (all expressed in 

 moles or molecules per liter, i.e., in equivalent concentrations). 

 For water, the formula becomes (brackets indicate concentra- 

 tions) 



_ [H+] X [QH-] 

 [HOH] 

 therefore 



i^[HOH] = [H+] X [0H-] 



The dissociation of water is so slight that in the product i^[HOH], 

 [HOH] is considered constant and is expressed with K as Kw, 

 the dissociation constant of water. The value of K for pure 

 water is IQ-^* (0.00000000000001). As this, in the case of water, 

 is the product of equal concentrations of [H+] and [0H~] ions, 

 the concentration of each of these must be 10"''. 



The dissociation constant of acetic acid [CHs-COOH] is 



In a normal solution of acetic acid, about 43 molecules in 10,000 

 are dissociated (at 25°C.). As each dissociated molecule yields 

 one hydrogen ion, the H+ ion concentration is 43/10,000 A^". 

 This is the numerical value of [H+] for acetic acid. The value 

 of [H+] for a normal solution of the very weak boric acid is 

 0.0000255 N. This means that the actual weight of free hydro- 

 gen ions in a normal solution of acetic acid is 0.0043 gram; and 

 in boric acid, 0.0000255 gram (in hydrochloric acid, it is 1 gram). 

 The concentration of hydrogen ions in any solution is obtained 

 by transposing the general formula, thus; 



K XC 

 Ci = • 



For water, this becomes 



[HOH] 



B.+ = K 



[OH- 



As the acidity of a solution decreases, the alkalinity increases, 

 which means that with a decrease in hydrogen ions there is a 

 corresponding increase in hydroxyl ions. Both reach a con- 

 venient limit in normal acid and alkaline solutions. These 

 limits constitute the ends of the hydrogen-ion scale and are 



