HW 1 - p 3.4 - Evaporation of Water into Air by Unimolecular Diffusion - 5 pts
Air at 25 oC with a dew point of 0 oC flows past the open end of a vertical tube filled with liquid water maintained at 25 oC. The tube has an inner diameter of 0.83 in., and the liquid level was originally 0.5 in. below the top of the tube. The diffusivity of water in air at 0oC is 0.256 cm^2/s.
posted by Dr. B at 2:14 PM
2 Comments:
So, I'm glad this whole thing is anonymous because I'm feeling rather stupid at the moment. I'm working this problem through and I'm using pressures in place of mole fractions. My question is this: I want to know the vapor pressure of water, or PA1. This is the area right above, or basically at the surface of the water. Can I assume the air here is saturated and that in turn, the partial pressure of component A, water, is equal to the saturation pressure of water at 25 deg C? Does that make sense?
Green Bucket 4:41 PM
Anonymous = good ! Otherwise no one asks questions.
I admit I solved the problem using mole fractions, but you can just as easily solve it using partial pressures.
I think you are mixing and matching partial pressure of water PA and vapor pressure of water PA*.
The connection is that at the surface of the water, the gas is saturated with water. Saturation = PA / PA* = 100%. So, PA at the surface of the water is equal to the vapor pressure of water....but at what temperature ? The answer is at T of the liquid, 25 degC.
Here is another hint..dew point is the T at which the gas WOULD be saturated.
Best of luck !
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