Volatile solutes

The same phenomenon occurs when two volatile--a compound that has an appreciable vapor pressure--components are mixed together. The only complication is that both materials can vaporize. The pressure inside the container is due to both liquids evaporating.

The partial pressures, P_a and P_b, are determined using Raoult's Law.

So, for example, methanol has a vapor pressure of 94 torr at 25 °C, and ethanol has a vapor pressure of 44 torr at 25 °C. Determine the vapor pressure of a solution made by mixing 1 mol of methanol and 1 mol of ethanol.

The vapor pressure of the solution is the sum of the vapor pressures of the solution's components.

The pressures are determine using Raoult's Law

What is the total vapor pressure inside the flask?

Notice that P_MeOH is greater than P_EtOH. Even though the solution is 50/50 ethanol/methanol the vapor contains more methanol than ethanol. As a matter of fact, we can determine the concentration of methanol and ethanol in the vapor phase. We do not know n_MeOH nor do we know n_EtOH, but we can find the molefraction of a in the vapor phase none-the-less. From the Ideal Gas Law we find that

huh?

So,

The mole fraction of EtOH in the vapor can be determined similarly.

There is more methanol in the vapor than ethanol. This makes sense because methanol is more volatile than ethanol (MeOH has a higher vapor pressure than EtOH) so the vapor should be enriched in MeOH.