In general, for an organic compound, a plot of the log vapor pressure versus inverse temperature is linear over a wide temperature range. The Clapeyron equation relates the log vapor pressure to the the inverse temperature, and is derived using our fundamental equation, thermodynamic definitions, and our criteria of phase equilibrium. It is common in the undergraduate curriculum to make a series of two assumptions to go from the Clapeyron to the Clausius/Clapeyron equation. First is it common to assume (1) that change in compressibility upon vaporization is approximately 1, or equivalently that the vapor phase may be treated as an ideal gas where the molar volume of the vapor is much greater than that of the liquid, which may be assumed negligible. And second , that the enthalpy of vaporization is constant. The resulting expression predicts that the log vapor pressure versus inverse temperature is linear, in apparent agreement with reference data .