In a mixture of two miscible liquids, how is the total vapor pressure calculated?

The total vapor pressure of a mixture of two miscible liquids is calculated by summing the individual vapor pressures of each component, which are weighted by their respective mole fractions in the mixture. This method is derived from Raoult's Law, which states that the partial vapor pressure of each component in a solution is proportional to its mole fraction in the liquid phase.

For each component, the partial vapor pressure is determined by multiplying the vapor pressure of that pure component by its mole fraction in the mixture. The total vapor pressure is then the sum of these partial pressures. This approach effectively captures how the presence of one liquid influences the evaporation of the other, leading to a total vapor pressure that reflects the composition of the mixture.

Using average values, multiplying pressures, or applying Dalton’s Law directly does not accurately represent the behavior of miscible liquid mixtures, particularly since the interactions between the components can alter the individual vapor pressures. Therefore, summing the individual vapor pressures based on their mole fractions provides the correct method for calculating the total vapor pressure in a miscible liquid mixture.