Question on GChem Chad Quiz

[leeaf]

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Can anyone help me out with this question?!!!;
Thank you in advance

If vapor pressure of pure water at 20 cel degree is 18 torr, what is the vapor pressure of water above a solution of 5.5m CH3OH?
ans: 16.4

i tried this question for a hour.. still can't get it....

 

[rnbaxter]

5.5 mol of methanol, it gives us molality so there are 1kg of H20
1000/18 mol of H2O

(1000/18)/(5.5 + 1000/18) multiplied by pure pressure (18) = 16.4