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                       FIGURE 12.3  Enthalpy-entropy (Mollier) diagram for water. (Source: Jones, J.B. and Dugan, R.E. 1996. Engineering
                       Thermodynamics. Prentice-Hall, Englewood Cliffs, NJ, based on data and formulations from Haar, L., Gallagher, J.S.,
                       and Kell, G.S. 1984. NBS/NRC Steam Tables. Hemisphere, Washington, D.C.)
                       Every equation of state is restricted to particular states. The realm of applicability is often indicated by
                       giving an interval of pressure, or density, where the equation can be expected to represent the p-v-T
                       behavior faithfully. For further discussion of equations of state see Reid and Sherwood (1966) and Reid
                       et al. (1987).
                       Ideal Gas Model
                       Inspection of the generalized compressibility chart, Fig. 12.5, shows that when p R  is small, and for many
                       states when T R  is large, the value of the compressibility factor Z is close to 1. In other words, for pressures
                       that are low relative to p c  , and for many states with temperatures high relative to T c , the compressibility
                       factor approaches a value of 1. Within the indicated limits, it may be assumed with reasonable accuracy
                       that Z = 1—i.e.,

                                                pv =  RT   or    pv =  RT                      (12.21a)

                       Other forms of this expression in common use are

                                                 pV =  nRT,   pV =  mRT                       (12.21b)

                       In these equations, n = m/M,  = M v, and the specific gas constant is R =  R/M  , where M denotes the
                                              v
                       molecular weight.

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