this problem has plagued oil reservoir engineers since the science was invented.
most of them simply treat every oil as a "black oil" (composed of methane which obeys the real gas law and heavy components which are liquid in the reservoir and stock tank). compositional models dont seem to get the job done either.
back in the old days, that was the mid '70's, texaco used fugacity(sp?)to try to calculate the part that would be liquid, that didnt work either.

Here is the explanation of Raoult's law and vapor pressure in ideal and real solutions.

Raoult's Law

Precisely.

If 10% of the molecules are butane, the vapor pressure of the resulting solution will be 10% that of pure butane, plus 90% of whatever pressure is contributed by other ingredients (of lower VP). There will be some deviation from this idel, but I believe hydrocarbons are pretty close to theoretical.

hydrocarbon mixtures probably behave as ideal components at relatively low temperature and pressure (i.e. atmospheric). not true at higher temperatures and pressures.