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I need help with chem, finding molar mass through vapor pressures?
At 25.0°C the vapor pressure of pure methanol (CH3OH) is 0.117 atm. If 6.61 g of a nonvolatile, nondissociating solute is dissolved in one mole of pure methanol, the vapor pressure of the solution at 25.0°C is 0.107 atm. What is the molar mass of the unknown solute
please help i tried a whole bunch of stuff and still couldnt get it :[
1 Answer
- ?Lv 71 decade agoFavorite Answer
The key concept is Raoult's Law:
The vapor pressure of a solution = vapor pressure of solvent x mole fraction of solvent.
(since there's only one volatile fraction.)
so 0.107atm = 0.117atm x mole fraction solvent.
mole fraction of solvent = 0.107atm / 0.117atm = 0.915.
1 mole = 0.915 x total moles.
Total moles = 1mole / 0.913 = 1.093moles.
moles solute = total moles - moles solvent = 1.093 - 1 = 0.093moles
6.61g = 0.093moles.
Molar mass = 6.61g / 0.093moles.
Source(s): http://en.wikipedia.org/wiki/Raoult's_law
