164 



ACIDITY 



their most outstanding property, their ability to react to form salts, 

 the molecules react according to the number of replaceable hydrogens of 

 acids and hydroxyl groups of bases. Note in the following equations 

 that the reacting powers per mole of HCl, H2SO4, and H3PO4 are in 

 the ratio 1:2:3. 



NaOH + HCl -^ NaCl + HoO 

 2NaOH + H2SO4 -> NaoS04 + 2HoO 

 3NaOH + H3PO4 -^ Na3P04 + SHoO 



Hence the complete reaction of one mole of sodium hydroxide requires 

 one mole of hydrochloric acid, but only one-half mole of sulfuric or one- 

 third mole of phosphoric acid. If these amounts of the respective acids 

 be diluted to a common volume, for example, 1000 ml., the resulting 

 solutions are of equivalent concentrations so far as their ability to react 

 with alkalies is concerned. It is upon such a basis that normal solu- 

 tions are prepared. 



By definition a normal solution is of such concentration that one liter 

 of the solution contains exactly one gram equivalent weight of the solute. 

 The gram equivalent weight is that weight of a compound that contains 

 one gram of replaceable (acid) hydrogen, or will react with one gram 

 of replaceable hydrogen, or is in any way equivalent to this weight of 

 hydrogen. To calculate the gram equivalent weight of acids divide the 

 gram molecular weight by the number of replaceable hydrogen atoms 

 in the molecule. Since one hydroxyl group requires one acid hydrogen 

 for its neutralization, it follows that the gram equivalent weight of 

 bases is obtained by dividing the gram molecular weight by the number 

 of hydroxyl groups in the molecule. For a salt the divisor is the number 

 of hydrogens that have been replaced in the formation of the salt from 

 the corresponding acid. Obviously, one obtains a like result in the last 

 two instances by dividing the gram molecular weight by the valence of 

 the metal contained therein. Thus the divisor for NaOH is 1, but for 

 CaS04 it is 2. The divisor in each case is termed the hydrogen equiva- 

 lent. The use of these values in calculations involving normal solutions 

 is illustrated in Table 7-1. 



One gram equivalent weight of a chemical substance is frequently 

 called simply an equivalent of that substance. One-thousandth of this 

 amount similarly is termed a milliequivalent (abbreviation m.e.q.), 

 which, if expressed in milligrams, is the same numerical figure as an 

 equivalent expressed in grams. For example, in the case of acetic acid 

 an equivalent is 60 g. (Table 7-1) and a milliequivalent is 60 mg. One 

 liter of a normal solution always contains one equivalent of the solute, 

 one milliliter containing one milliequivalent. Analogous fractions of a 

 mole are also frequently used in biochemical work. Thus, one-thousandth 

 of a mole is a milli^nole, and one-millionth is a micromole. Molarity 



