At 70°C and 100 kPa, both ethanol and ethyl acetate exist in the liquid phase. As the temperature increases, the vapor pressure of ethanol increases and it will evaporate at its boiling point. At 80°C, there will be a mixture of vapor and liquid phases, with the vapor having a higher mole fraction of ethanol.
The mixture consists of ethanol (C₂H₅OH) and ethyl acetate (CH₃CO₂C₂H₅), with a mole fraction of ethanol (X₁) equal to 0.20. To describe the evolution of phases and phase compositions as the temperature is gradually increased to 80°C, we need to examine the vapor pressure-temperature relationship for both components.
At 70°C and 100 kPa, both components exist in the liquid phase. As the temperature increases, the vapor pressure of ethanol increases due to its higher volatility compared to ethyl acetate. At a certain temperature, the vapor pressure of ethanol will reach the overall system pressure and it will start to evaporate. This temperature is known as the boiling point of ethanol.
Since the boiling point of ethanol is lower than that of ethyl acetate, the composition of the vapor phase will be enriched in ethanol as it evaporates. At 80°C, you can expect to have a mixture of vapor and liquid phases, with the vapor phase containing a higher mole fraction of ethanol compared to the liquid phase.