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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.

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If the vapor (i.e. gas) phase above the solution can be treated as an ideal gas, then yes, if two species have the same partial pressure, then they have the same mole fraction.

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.

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  • $\begingroup$ By the way, the way you defined the equation is different from the way Raoult's law is, no? I didn't know you could consider yi for vapor phase also, I thought that it was only valid for the solution $\endgroup$
    – copper
    Commented Feb 19, 2015 at 0:17
  • $\begingroup$ Yes, so (i) if Raoult's law is valid, and (ii) the vapor phase is ideal (those are two separate assumptions), then $y_i P = x_i p^*_i$. $\endgroup$
    – Curt F.
    Commented Feb 19, 2015 at 0:18
  • $\begingroup$ Ok, because I just noticed that I have an exercice that changes a bit the definition of raoult's law the way you did it. I wasn't sure how I was supposed to understand it... Thank you $\endgroup$
    – copper
    Commented Feb 19, 2015 at 0:21

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