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.

Question

a)
What are the mole fractions of ethanol and solute?
b)
What is the molecular weight of the solute?

Answer 1

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*P^o_solvent
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.