WILLIAM D. HARKINS 145 



While the activity of a perfect gas, or a substance which in solution obeys the per- 

 fect-gas law, is expressed by its concentration, this is not the case if the gas law does 

 not hold. The activity may then be defined as that quantity which, when substituted 

 for the concentration of a substance in the mass-law equation, expresses its effect in 

 determining the equilibrium. 



If a small quantity of iodine is shaken with water and carbon disulphide at 25° C. 

 until equilibrium is attained, it is found that a unit volume of carbon disulphide con- 

 tains six hundred times more iodine than the same volume of water. Since there is 

 equilibrium, the tendency of iodine to escape from the water is the same as that from 

 carbon disulphide, i.e., the vapor pressure of iodine should be the same over the two 

 solutions, and this is found to be true. The identity of vapor pressures of the iodine 

 indicates that the activities are equal. 



If it is considered that the activity of the iodine in the carbon disulphide is equal 

 to its concentration, then the activity in water, which is the same, must be six hundred 

 times the concentration of the iodine in the water. Thus carbon disulphide is a better 

 solvent than water for iodine since it can hold six hundred times as much of the latter 

 and still give no more activity to the iodine.' 



1. When the activity of a constituent is the same in two difTerent phases the con- 

 stituent will not increase its concentration in one phase at the expense of the other un- 

 less energy is supplied to effect the transfer. 



2. If the activity of a constituent is greater in one phase than another the con- 

 stituent (if transferred at all) will pass from the phase in which it has the greater into 

 the one in which it has the lesser activity. 



THE ACTIVITY COEFFICIENT OF ELECTROLYTES 



A solution of common salt which contains 58.5 gm. of sodium chloride to 1,000 gm, 

 of water is considered to be i molal in concentration. If m represents the molality of 

 a solution, its activity coefficient (a) is defined as the ratio of its activity to its mo- 

 lality, or 



a 

 a = — ; so a = ma . 



m 



The activity coefficient for a salt in its extremely dilute solution is i.oooo by defi- 

 nition. At a molality of o.oi the activity coefficient for sodium chloride in its aqueous 

 solution is 0.922, while at o.im it is 0.798. Now the activity coefficient plays the same 



' The escaping tendency may be expressed also in terms of the "fugacity" (/), a term introduced 

 by G. N. Lewis. The fugacity of an ideal gas is equal to its pressure. Any gas is practically ideal at 

 low pressures. At higher pressures its fugacity is the geometric mean of the actual pressure {P) of the 

 gas and the ideal pressure {Pi) calculated from the gas law, so that 



^ Pi 



The activity of a constituent may now be defined as its relative fugacity (/) as compared with its 

 fugacity in some standard state (/J, or 



