Thermodynamics - dependence of molar fraction on temperature

[sari]

Why does the molar fraction of a solution not depend on temperature?

In other words, why will heating a solution preserve its composition? It seems to me that if I have two substances that evaporate at different rates, the substance with the faster evaporation rate will pass more moles into the gas phase, so the proportions of the substances in the solution should not be preserved.

 

[sari]

After thinking about it more, I actually found that the molar fraction does depend on the temperature (funny, because another online source mislead me by stating that it does not depend on the temperature).

If you know the initial composition, the enthalpies of evaporation for each substance and you know the quotient of the partial pressures after heating you can use Raoult's law to find the composition after heating.