PAPERS OX CHEMISTRY AND PHYSICS 297 



THE VAPOE PEESSUEE AND HEAT OF VAPOEI- 

 ZATIOX OF XOX-ASSOCIATED LIQUIDS. 



F. S. ^loRTiMER^ Illixois TTesletax Uxtveesity 



lu the application of physico-chemical principles to in- 

 dustrial and maniifactnring processes, few questions are 

 of more frequent occurrence than those dealing with dis- 

 tillation problems. T*he questions take a variety of 

 forms, but most of them may be answered when the 

 vapor pressures of the substances involved are known. 

 Occasionally the desired information may be found in, 

 or calculated directly, from data given in the literature. 

 More commonly this is not the case, and it is then neces- 

 sary either to make the measurements directly or to re- 

 sort to some method of calculation of the required data. 



Many expressions have been developed for calculating 

 vapor pressures and heats of vaporization, probably the 

 most useful of which is that obtained from the integra- 

 tion of the Clausius-Clapeyron equation. Assuming the 

 gas laws to apply and also assuming that the heat of 

 vaporization is a constant throughout the desired range, 

 and introducing common logarithms, there is obtained 

 the expression: 



-L 



Log P = ^C (1) 



4.5S T 



in which P is the vapor pressure and T is the absolute 

 temperature at which P is measured. C is an integra- 

 tion constant, the value of which depends upon the units 

 used. 



Equation (1) indicates that if the common logarithm 

 of the vapor pressure be plotted against the reciprocal 

 of the absolute temperature, a straight line should re- 

 sult. In view of the rather bold assumptions concerning 

 the applicability of the gas laws and the constancy of the 

 latent heat of vaporization, it is quite remarkable how 

 closely experimental data may be represented by this 

 linear equation throughout relatively wide ranges of 

 temperature. 



In equation (1) the term L/4.58 represents the slope 

 of the log P vs IT vapor pressure curve. The Integra- 



