A further discussion is provided by Elliott and Lira . In that work the authors arrive at eq. ( ) using the assumptions listed earlier. However, this is followed by the subsection 9.3 "Shortcut Estimation of Saturation Properties." Here the author mention that one may alternatively arrive at eq. ( ) by instead assuming that ∆H vap /∆Z vap is constant. Further, this assumption is reasonable over the range 0.5 < T r < 1, where T r = T/T c is the reduced temperature, where T c is the critical temperature. The authors then use the critical point (T r = 1) and accentric factor (T r = 0.7) as reference points to solve for the constants A and B, to develop a predictive vapor pressure expression. We note also that the suggestion of assuming ∆H vap /∆Z vap is constant is also made in references 18 and 17; however, these are references not typically used in undergraduate courses.