Chemistry Ap

At 20◦C the vapor pressure of pure methanol
(CH3OH) is 89.0 torr and of pure water (H2O)
is 17.5 torr. A solution of methanol and water
has a total vapor pressure of 35.5 torr at
20◦C. Calculate the mole fraction of methanol
in this solution. Assume the solution obeys
Raoult’s Law

I haven't a clue what to do for this problem. Wasn't in class for example problems so I don't know what goes with what to determine what.

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  1. We can't draw diagrams on this forum but it is easy to see if I could draw a graph.
    Basically this is what you do.
    XH2O = Pomethanol-Ptotal/(Pomethanol-PoH2O)
    That gives you (89.0-35.5)/(89.0 - 17.5) = 0.748 for H2O

    I can never remember the formula; I draw it out with a graph and do it by proportion.
    Then Xmethanol = 1-XH2O or you can do it for methanol first as
    Xme = (Ptotal-PoH2O)/(Pomethanol - PoH2O) . That should give you the same answer as 1-0.748 = ?

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