In this chapter, we will relate thermodynamic properties that cannot be measured, such as enthalpy, internal energy, entropy, and specific heats, to properties that can be measured: temperature, pressure, mass, and volume. In previous chapters, tables have been used to determine properties of interest. How were all those properties determined? This chapter will provide those relationships that were used to determine properties over the wide range of temperatures and pressures encountered in the various devices analyzed in our study. In addition, the relationships developed in this chapter will allow us to perform more accurate calculations of properties than those used in previous chapters.