posted by Anonymous .
When 9.18 g of a soluble nonelectrolyte was dissolved in 100 g of ethanol at 78.4°C, the vapor pressure of the solution was measured to be 740 mm Hg.
What are the mole fractions of ethanol and solute?
What is the molecular weight of the solute?
I don't know the vapor pressure of ethanol at 78.4 C but you can look that up. I will assume it is 760 mm since I know that is close to the boiling point of ethanol.
Psoln = Xsolvent*Posolvent
Solve for Xsolvent.
Then 1-Xsolvent = Xsolute which is part a.
Then Xsolute = (moles solute/moles solute + moles solvent).
You know moles solvent = 100 g/molar mass ethanol. Substitute into the Xsolute equation above and solve for moles solute.
Then n(solute) = grams solute/molar mass
You have n and g, solve for molar mass, which is part b.