chemistry
posted by regina .
Dissolve 1.5g of an unknown salt into 37g of water and find that the new vapor pressure at 75C is 277 torr. What is the Molar Mass of the unknown? How many grams of the unknown would have to be used in 100ml of water to make a 11.5% solution by mass of the unknown?

Psoln = XH2O*P^{o}_{H2O}
Look up Po for H2O at 75, substitute 277 torr for Psoln and solve for XH2O.
Then XH2O = (moles H2O)/(moles H2O + mols Unk).
Solve for moles Unknown
Then moles = grams/molar mass and solve for molar mass.
Probably you can pick it up here and obtain the % by mass solution. Post your work if you get stuck. 
thoo