chemistry

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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 -

    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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