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Topic: phase change problem  (Read 2642 times)

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Offline eunChae

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phase change problem
« on: May 21, 2009, 06:17:41 AM »
When i was studying my exam, i encountered this problem:
A 1 g sample of H2O(g) is injected into a 4 L flask at a temperature of 35 C and cooled. To the nearest degree Celsius, determine the temperature at which the first liquid water will condense.
Now, I should emphasize that I m not a native speaker of English so i couldnt have understood the problem correctly but as i understood, at the conditions above, by the ideal gas equation the pressure of H2O(g)  at 35 C is approximately 26.8 mmHg. My question is how can we liquefy H2O just by cooling it ? Because according to the phase diagram, we should also apply some pressure.
The answer of the problem above is 27 C. What should be my approach to solve this prob.?
Thanks in advance... 
eunChae
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Offline Borek

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Re: phase change problem
« Reply #1 on: May 21, 2009, 07:10:32 AM »
At what temperature saturated vapor pressure is 26.8 mm Hg?
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Offline eunChae

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Re: phase change problem
« Reply #2 on: May 21, 2009, 07:22:20 AM »
I got your point Borek!! The first time H2O(g) liquefies, we should consider the vapor pressure of water. Checking my book, the vapor pressure is 26.7 at 27 Celsius confirming the answer.
Thank you very much.
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