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Unformatted text preview: CHEMISTRY 201X: Homework #4 "Phase Diagrams and Transitions; Vapor Pressure" Due: Monday, October 1, 2007 (1) (16 points) (a) (4 pts) Using the following information, sketch the phase diagram for ammonia for pressures between 0 and ca. 1.2 atm and for temperatures between 0 and 300 K. Label important points on this curve and phases existing in each region. Normal melting point = -78C Normal boiling point = -33.5C Triple Point Temperature = -78.2C Pressure at Triple Point (s, l, g) = 45.6 torr Heat of Fusion = 5.65 kJ/mol Heat of Vaporization = 23.36 kJ/mol Density of NH3(s) near n.m.p. = 0.833 g/cm3 Density of NH3(l) near n.b.p. = 0.682 g/cm3 (b) (2 pts) Use the Clausius-Clapeyron equation to predict the pressure at the triple point given the normal boiling point of ammonia and the triple point temperature. Compare your result with experiment. (c) (4 pts) Assuming that the densities of liquid and solid ammonia do not change with temperature or pressure, estimate the slope of the line for equilibrium between solid and liquid phases. You may assume that the solid liquid equilibrium line remains straight. dp dT s =
l (s) (l) T MW (s) - (l) H fus (d) (4 pts) The critical density for ammonia is 0.225 g/cm3. Calculate the critical molar volume of NH3 in units of L/mol. According to the van der Waals equation of state, the critical molar volume is Vc = 3b, where b is the the van der Waals "b" parameter. Use your answer to determine a and b for ammonia based on the experimental data (critical temperature = 132.4C; critical pressure = 111.9 atm). Expressions for critical temperature and pressure are 8a a . Tc = ; pc = 27 Rb 27b2 Compare your results of a and b with values you can find from standard tables (you may look in textbooks or the CRC Handbook). (e) (2 pts) Use the critical point and the normal boiling point to estimate the heat of vaporization for ammonia. Offer an explanation for any difference with the value given above. (2) (12 points) (a) (4 pts) 10.00 L of dry air are bubbled slowly through liquid water at 20.00C and the observed mass loss of the liquid is 0.1726 g. Assume that 10.00 liters of saturated water vapor were formed in the experiment. Calculate the vapor pressure of water at 20.00C from this experiment. You may assume that water vapor is an ideal gas. (b) (4 pts) The same experiment at 50.00C gives an observed mass loss of the liquid of 0.8258 g. Calculate the vapor pressure of water at 50.00C from this experiment. (c) (4 pts) Using your results from (a) and (b), estimate the heat of vaporization of water (in kJ/mol). Look up heats of vaporization for water at 0C and 100C. How does your estimate compare with these values and can you explain the differences? ...
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