[schu9600]

 The dissolution of 5.25 g of a substance in 565 g of benzene at 298 K raises the boiling
point by 0.625°C.  Note that Kf = 5.12 K kg mol-1, and Kb = 2.53 K kg mol-1, and the density of benzene is 876.6 kg m-3.  Calculate the freezing point depression, the ratio of the vapor pressure above the solution to that of the pure solvent, the osmotic pressure,
and the molecular weight of the solute.          
 
    Pressure benzene = Torr at 298 K

  :delta: T = K_fb  (Freezing point depression formula)

For the first part I'm assuming I need to find Delta T and that is the freezing point depression but do not know how to figure out the molality (b) of the unknown substance. I know it is moles of solute per kilograms solvent but dont know how to determine the number of moles.

Any help you could give me on this question would be greatly appreciated.
Thanks for your time  

[Borek]

Use boiling-point elevation to calculate molality, knowing molality and mass of substance calculate molar mass.

[schu9600]

Okay so I've determined the freezing point depression and the molecular weight, and the osmotic pressure.  I am just wondering now how to find the ratio of vapour pressure above the solution to that of the pure solvent.  I initially thought to find the molar fraction of the solute and use Henry's law but that result doesn't seem to be correct.

Any tips?