Humid air is enclosed in a 2.00-liter flask at 40C. The flask is slowly cooled. When the

Chapter 8, Problem 8.85

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Humid air is enclosed in a 2.00-liter flask at 40°C. The flask is slowly cooled. When the temperature reaches 20°C, drops of moisture become visible on the flask wall. Although the pressure in the flask changes when the temperature drops, it remains close enough to 1 atm for the psychrometric chart to provide a close representation of the behavior of the system throughout the process. Use the chart to solve the following problems.

(a) What were the relative humidity, absolute humidity, and wet-bulb temperature of the air at 40°C?

(b) Calculate the mass of the water in the flask. (See Example 8.4-5.)

(c) Calculate the enthalpy change in joules undergone by the air in going from 40°C to 20°C.

(d) Write an energy balance for this closed-system process, taking the wet air in the flask as the system, and use it to calculate the heat in joules that must be transferred from the air to accomplish the cooling. (Assume ideal-gas behavior, so that \(\hat{H}=\hat{U}+R T\).)

(e) You assumed that the pressure in the flask remained constant at 1 atm throughout the cooling process so you could use the psychrometric chart. What would the pressure in the flask actually have done, and why? How would your answers to Part (a) change if you took the pressure change into account (increase, decrease, no change, can't tell without more information)? Explain your answers.