Homework Help: chemistry

Posted by meghan on Thursday, December 2, 2010 at 12:08am.

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)

chemistry - DrBob222, Thursday, December 2, 2010 at 1:11am
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.

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