From: glhurst@onr.com (Gerald L. Hurst) Newsgroups: sci.physics Subject: Re: vapor pressure of H2O Date: 13 Jan 1996 22:51:33 GMT In article <DL4rqG.oB@iglou.com>, rooster@iglou.iglou.com (Rooster) says: You can find equations or tables in the handbooks (CRC, Lange), but it might be more educational to note that pure compounds tend to give linear plots of LogP versus 1/T where P is the vapor pressure of the material at T (Kelvin or absolute temperature). Since LogP = K*1/T + C, you can determine the value of K as the slope of the line between the points at the freezing and boiling points of water. These two values are 4.579 and 760.0 mm respectively. Check your new equation at 50 degC (323K) to see how close it comes to the correct value. You should come within 5-6% of the true answer. Handbook equations massage this basic equation to get a more reliable empirical equation, usually starting by replacing the true absolute temperature with a fudged quantity, (T+a), where a is whatever constant yields the most reliable set of K and C values. Now that microcomputers are available, this tedious trial and error search for suitable constants should be childs play for the students. Jerry (Ico)

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