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Hence if the molar mass increases by a factor of 4 at

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Unformatted text preview: and the temperature remain constant. This is true and corresponds to the horizontal section of the condensation isotherm in a P vs. Vm plot, where the liquid and vapor phases coexist. Think also about the boiling of water at ambient pressure. b) A unique molar volume characterizes the system at all stages of the condensation of a gas into a liquid. This is incorrect. Indeed while the pressure and the temperature remain constant during condensation, indicating that two phases are in equilibrium, two different molar volumes are needed to characterize the system. These are the molar volume of the liquid phase and the molar volume of the vapor phase. c) The compressibility factor is larger than one above the Boyle’s temperature and less than one below. This statement is also incorrect. The statement would have been correct if the word compressibility had been replaced by the word compression. Remember Z > 1 when repulsive forces dominate. d) On a compression isotherm, the first droplet of liquid is observed at a molar volume higher than the critical molar volume. This is true, since the critical point is located at a molar volume that is always intermediate between the molar volumes of the liquid and vapor phases in equilibrium with each other at any temperature. Question 7: (5 points) Indicate which of the following statements is incorrect: a) In a mixture A/B of ideal gases, the ratio of partial pressures PA/PB is equal to the ratio of the mole fractions xA/xB This is correct and a result of Dalton’s law. The partial pressure of a component A in an ideal gas mixture is given as PA = xAP where xA is the mole fraction of A in the mixture and P is the total pressure. Hence, applying this equation to two components A and B of a mixture leads to PA/PB = xAP/xBP = xA/xB b) The relationship between ΔH and ΔU for a chemical reaction occurring at constant T is given by: ΔH = ΔU + RT Δng This is correct and the direct result of H = U + PV applied to the products and the reactants in a gas reaction. ΔH = ΔU + Δ(PV) 3 The difference Δ(PV) for a chemical reaction occurring at constant temperature T is equal to Δ(ngasRT) = RT Δ(ngas). Note that we are only considering the gas species in...
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