366 KEYES 



ART. J 



rule and some of its consequences were developed. There are 

 essentially three effects which it is necessary to consider in order 

 to use the method for the exact determination of vapor pres- 

 sures. First, the effect of the pressure of the neutral gas on the 

 vapor pressure of the liquid must be determined. This is the 

 Poynting effect and has already been sufficiently discussed. 

 Second, the depression of the vapor pressure of the liquid due 

 to the dissolved gas must be computed. If, as usual, the 

 solubility is slight, as with water at zero degrees saturated with 

 air at atmospheric pressure, the change in vapor pressure due 

 to solubility is neghgible. Third, Dal ton's law in the form 



usually applied, pi = Xip or pi = - — p (Gibbs' notation, c.f . 



[298]), where x is the mol fraction, is inexact. The example to 

 follow will illustrate the use of the Gibbs-Dalton rule, p = 2pi. 



The third correction may be made by using the latter rule, 

 or we require actual experimental data relative to the p, v, t 

 behavior for the mixtures of interest and the neutral gas. 

 Equivalent to the latter data is a knowledge of the constants of 

 the equation of state for the two gases (gas emitted by liquid 

 and neutral gas) together with the law of combination of the 

 constants of the equation of state^^ to give the properties of 

 mixtures. Enough knowledge of the latter sort is available to 

 be useful in many cases. 



As a concrete problem, suppose an aqueous salt solution at 

 the fixed temperature 21.2° is in equilibrium with nitrogen, the 

 total pressure of the gaseous mixture being one atmosphere. 

 Let the water vapor be absorbed and weighed while the nitrogen 

 is passed along to be measured for pressure and volume at 25°C. 

 The weight of the water is 0.45 gram or 0.02498 mols, and the 

 nitrogen has a volume of 24000 c.c. at 1 atm., or 0.98111 mols. 

 The perfect gas law is suitable for computing the latter since 

 nitrogen is very nearly a perfect gas at 25° and 1 atm. The 

 constants jS and A of the equation of state (Vila) for water and 

 nitrogen* are 



* The constants given for water are only approximate. Those for 

 nitrogen are valid for low pressures at ordinary temperatures. This is 

 not the place for a complete and exact exposition of the theory of reduc- 



