posted by meghan on .
a student needs to prepare an aqueous solution of sucrose at a temperature of 20 degrees c with a vapor pressure of 15. 0 mm Hg. How many grams of sucrose (mm=342 g/mol) does she need if she uses 375 g H20? (the vapor pressure of water at 20 degrees c is 17.5 mm Hg)
Psoln = XH2O*PoH2O
15.0 torr = XH2O*17.5 torr
Solve for XH2O which gives you the mole fraction that H2O must be. I'll call that ??
[molesH2O/(moles H2O + moles sucrose)]=??
moles H2O = grams water/molar mass H2O, substitute this into the equation immediately above this (both numerator and denominator) and solve for moles sucrose.
Finally, moles sucrose = grams/molar mass sucrose.
Then check everything to make sure all of that gives you 15.0 torr.