Which equation assumes that the vapor phase is ideal in a two-phase system?

The Clausius-Clapeyron equation specifically addresses the relationship between the temperature and pressure of a phase transition, particularly where a substance changes between liquid and vapor phases. This equation assumes that the vapor phase behaves ideally, which allows for a simplified analysis of the equilibrium between the two phases.

In a two-phase system, the ideal behavior of the vapor means that the interactions between the gas molecules are minimal, and their properties can be described accurately using the ideal gas law. The Clausius-Clapeyron equation leverages this assumption to relate changes in pressure and temperature to the latent heat of vaporization or sublimation. This utility is vital for understanding phase transitions in thermodynamic processes.

While the Clapeyron equation can also describe phase equilibria, it does not explicitly incorporate the assumption regarding the ideality of the vapor phase as clearly as the Clausius-Clapeyron equation. Other options provided, such as Hess's law and the ideal gas law, pertain to different concepts in thermodynamics and do not directly focus on phase changes or the interaction between liquid and vapor phases.