# Pressure over a solution

I was wondering something, if I have the same molar fraction for two different substances for a vapor on top of a solution, will this mean that the pressure of each individual substance will be the same ? Since they have the same quantity of molecules ?

This question has a link with Raoult's law : $p_{solvent}=x_{solvent}p^*_{solvent}$, where $p_{solvent}$ is the partial pressure above the solvent, $x_{solvent}$ is the mole fraction of solvent in the mixture, and $p^*_{solvent}$ is the vapor pressure of pure solvent.

For an ideal gas mixture, $p_i = y_i P$, where $p_i$ is the partial pressure of species $i$ and $y_i$ is the mole fraction of $i$ in the vapor phase, and $P$ is the total pressure.
The ideal gas assumption will be very accurate in many cases commonly encountered in chemistry, but not always. Binary mixtures of $\ce{CO2}$ and another compound at high pressure would be one example of vapor phases that are not ideal. Concentrated vapor of aliphatic carboxylic acids is also not very ideal.
• Yes, so (i) if Raoult's law is valid, and (ii) the vapor phase is ideal (those are two separate assumptions), then $y_i P = x_i p^*_i$. – Curt F. Feb 19 '15 at 0:18