Homework Help: P. Chemistry

Posted by Tommy on Sunday, March 7, 2010 at 10:53pm.

Can anyone direct me on how to solve this problem? All of the variables are makin my head spin--not sure how to organize them. If you can give equations to use that would help.Thnx!

1 L of an impure water solution has a freezing point of -11 °C. Which of the following is the
best estimate of the vapor pressure of the solution at 25 °C? Assume the vapor pressure of pure
water at 25oC = 23.76 atm, kF = 2 (K kg)/mol, and that water is 55 mol/L. Assume the density of
water is 1 g/mL.

P. Chemistry - DrBob222, Sunday, March 7, 2010 at 11:08pm
delta T = Kf*molality
11 = Kf*m
Calculate m

molality = moles/kg solvent.
I think we must assume 1 kg solvent although that won't be exactly right.
Calculate (estimate) # moles.

#moles + 55.5 for H2O = total moles.
Calculate mole fraction water,
Then vapor pressure H2O = mole fraction water x 23.76 BUT I don't think that is atm (It must be torr or mm Hg). This would be an estimate at best. Check my thinking.

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