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Unformatted text preview: 3-46Special Topic: Vapor Pressure and Phase Equilibrium3-102 A glass of water is left in a room. The vapor pressures at the free surface of the water and in the room far from the glass are to be determined. AssumptionsThe water in the glass is at a uniform temperature. PropertiesThe saturation pressure of water is 2.339 kPa at 20C, and 1.706 kPa at 15C (Table A-4). AnalysisThe vapor pressure at the water surface is the saturation pressure of water at the water temperature, H2O 15C kPa1.706===Csat@15@satsurfacewater ,waterPPPTvNoting that the air in the room is not saturated, the vapor pressure in the room far from the glass is kPa1.404====kPa)339.2)(6.(Csat@20@satair,airPPPTv3-103The vapor pressure in the air at the beach when the air temperature is 30C is claimed to be 5.2 kPa. The validity of this claim is to be evaluated. PropertiesThe saturation pressure of water at 30C is 4.247 kPa (Table A-4). WATER 30CAnalysisThe maximum vapor pressure in the air is the saturation pressure of water at the given temperature, which is kPa4.247===Csat@30@satmax,airPPPTvwhich is less than the claimed value of 5.2 kPa. Therefore, the claim is false. 3-104The temperature and relative humidity of air over a swimming pool are given. The water temperature of the swimming pool when phase equilibrium conditions are established is to be determined. AssumptionsThe temperature and relative humidity of air over the pool remain constant. PropertiesThe saturation pressure of water at 20C is 2.339 kPa (Table A-4). AnalysisThe vapor pressure of air over the swimming pool is Patm, 20C kPa0.9357kPa)339.2)(4.(Csat@20@satair,air====PPPTvPOOL Phase equilibrium will be established when the vapor pressure at the water surface equals the vapor pressure of air far from the surface. Therefore, kPa0.9357air,surfacewater ,==vvPPand C6.0===kPa9357.@sat@satwatervTTTPDiscussion Note that the water temperature drops to 6.0C in an environment at 20C when phase equilibrium is established. 3-473-105Two rooms are identical except that they are maintained at different temperatures and relative humidities. The room that contains more moisture is to be determined. PropertiesThe saturation pressure of water is 2.339 kPa at 20C, and 4.247 kPa at 30C (Table A-4). AnalysisThe vapor pressures in the two rooms are Room 1: kPa1.699====kPa)247.4)(4.(Csat@301@sat111PPPTvRoom 2: kPa1.637====kPa)339.2)(7.(Csat@202@sat222PPPTvTherefore, room 1 at 30C and 40% relative humidity contains more moisture. 3-106EA thermos bottle half-filled with water is left open to air in a room at a specified temperature and pressure. The temperature of water when phase equilibrium is established is to be determined....
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- Spring '08