# CHEM 341 Susquehanna University Physical Chemistry Questions

Chem 341Fall 2021
HW #6 (due 10/11/21)
1. Freeze drying is a water removal process typically used to preserve perishable materials, to
extend shelf life or make the material more convenient for transport. Freeze drying works by
freezing the material, then reducing the pressure and adding heat to allow the frozen water in the
material to sublime. (https://www.millrocktech.com/lyosight/lyobrary/what-is-freeze-drying)
The heat of sublimation of a substance is the sum of the heats of fusion and vaporization.
You have collected a tissue specimen that you would like to preserve by freeze drying. To ensure
the integrity of the specimen, the temperature should not exceed -8.00 °C. The vapor pressure of
ice at 273.16 K is 624 Pa. What is the maximum pressure at which the freeze drying can be carried
out? The enthalpies of fusion and vaporization of water are 6.01 kJ/mol and 40.66 kJ/mol,
respectively.
2. An ideal solution of 5.00 mol of benzene and 3.25 mol of toluene is placed in a piston and
cylinder assembly. At 298 K, the vapor pressure of the pure substance are P*benzene = 96.4 Torr and
P*toluene = 28.9 Torr.
(a) Calculate the total vapor pressure above the mixture.
(b) Calculate the mole fractions of the two components in the vapor phase.
3. A system contains 1.00 mg of ClF gas (molar mass = 54.45 g/mol). A mass spectrometer
separates the gas into the species 35ClF and 37ClF. Calculate ∆S and ∆G at 500. K. Isotopic
abundances: 19F = 100%; 35Cl = 75.8%; 37Cl = 24.2%.
4. The dissolution of 5.25 g of a solute in 565 g of benzene at 298 K raises the boiling point by
0.625 °C. Calculate the following:
(a) The molality of the solution. The boiling point elevation constant for benzene is 2.53 °C kg
mol-1.
(b) The freezing point depression of benzene. The freezing point depression constant for benzene
is 5.12 °C kg mol-1.
(c) The molar mass of the solute.
(d) The osmotic pressure at 298 K, assuming the volume of the solution is the same as the volume
of benzene. The density of benzene is 876.6 kg m-3.

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