294 PROTOPLASM 



weight of the acid is 98; of this amount, two are hydrogen; 

 therefore one-half of the molecular weight, or 49 grams, in a 

 liter will give 1 gram of hydrogen per liter. Acetic acid has 

 four hydrogen atoms in its molecule (CH3COOH), but only one 

 of these — the H of the carboxyl group (COOH) — can be replaced 

 by the metal (Na) of a base (NaOH). Consequently, to make 

 up a normal solution of acetic acid, the whole molecular weight 

 in grams (60) must be taken, the hydrogen atoms in the CH3 

 group being ignored because they are not replaceable by the 

 metal of a base. We must, therefore, when making up a normal 

 solution consider whether the acid is monobasic, like hydro- 

 chloric, or dibasic, like sulphuric; we must also consider the 

 total molecular weight, including any water of crystallization; 

 and finally consider what part of the hydrogen is replaceable by a 

 base. 



A normal solution of a base is like that of an acid, except that 

 we think in terms of the hydroxyl (OH) group instead of the 

 hydrogen atom. 



Acidity of the normality type, which we have been considering, 

 is capable of measurement by titration; that is to say, the quantity 

 of replaceable hydrogen can be determined by measuring the 

 quantity of alkali that is exactly necessary to neutralize the 

 solution. A color indicator (dye) may be used to determine 

 the exact point of equilibrium between the acid and the base. 

 Thus, if a dye such as litmus or phenolphthalein is added to an 

 alkaline solution, the color is blue in the case of litmus and red in 

 phenolphthalein. If, now, acid is added drop by drop, the 

 amount being measured vmtil the solution just turns in color 

 (from blue to red in litmus and from red to colorless in phenol- 

 phthalein), then the solution is at (or near) the point of neutrality, 

 and the amount of acid added just balances the alkali present. 



Molarity. — The convenience of expressing concentrations in 

 one way or another depends on our interest. If we are thinking 

 of a solution as a physical system, capable of exerting, for 

 example, osmotic pressure, then the total number of molecules 

 (or ions) present is important. In this case, a concentration 

 expressed in terms of total molecules is more helpful than one 

 in which the concentration of hydrogen only is given. The 

 physical chemist, therefore, uses normality less as a means of 

 expressing concentration and molarity more. A molar solution 



