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Q: Vapor Pressure & Temperature ( Answered,   0 Comments )
Question  
Subject: Vapor Pressure & Temperature
Category: Science > Chemistry
Asked by: cdepaola-ga
List Price: $20.00
Posted: 16 Feb 2006 06:45 PST
Expires: 18 Mar 2006 06:45 PST
Question ID: 446510
If we know the vapor pressure of a liquid at a given temperature can
we then find the new vapor pressure of the same liquid at a different
temperature?

That is say our liquid is Toluene and has a VP of 22 @ 20C (68F) can
we find the new VP at say 0C or 40C?
Answer  
Subject: Re: Vapor Pressure & Temperature
Answered By: guillermo-ga on 16 Feb 2006 21:56 PST
 
Hello Cdepaola-ga,

Yes, you can.

There are two common methods to calculate the vapor pressure for
different temperatures.

One is the Clausius-Clapeyron equation, to estimate the vapor
pressures of pure liquids or solids parting from the vapor pressure
for a temperature already known. Since this only-text platform is not
very friendly to post mathematical formulas, please follow this link
where you can see the equation and its references:
http://antoine.frostburg.edu/chem/senese/101/liquids/faq/clausius-clapeyron-vapor-pressure.shtml
(1)

While it works for most applications and is easy to derive and justify
theoretically, it's known that this method fails at high pressure and
near the critical point (2), and under these conditions its results
will be incorrect.

Thus, for more reliable estimates, chemist engineers prefer the
Antoine equation, which requires to know the Antoine coefficients.
Please see the equation at
http://antoine.frostburg.edu/chem/senese/101/liquids/faq/antoine-vapor-pressure.shtml
(1) At the same page you'll also find resources for Antoine
parameters.

So far, the affirmative answer to your question and the methods to do
it. Now, for a simplified way there is an online calculator by Shuzo
Ohe, Ph. D, for a number of substances -- toluene included -- which
claims to be based on Antoine parameters:
http://www.s-ohe.com/Vp_calc.html For toluene the calculation page is
http://www.s-ohe.com/Toluene_cal.html

Entering a temperature of 20 C, the resulting VP is 21.86 [mmHg]
(aproximately 22, as you posted). At 0 C, it would be 6.74 [mmHg];
and at 40 C, 59.18 [mmHg].

______________________________________
(1) Source: General Chemistry Online
(2) Critical point: "The thermodynamic state in which liquid and gas
phases of a substance coexist in equilibrium at the highest possible
temperature. At higher temperatures than the critical no liquid phase
can exist." http://amsglossary.allenpress.com/glossary/browse?s=c&p=105
(Glossary of Meteorology - American Meteorological Society)
______________________________________

I hope you find this information complete enough. Otherwise, or if
there's something not totally clear, please ask for clarification and
I'll be glad to respond it. Thanks for your question.

Sincerely,

Guillermo

Clarification of Answer by guillermo-ga on 16 Feb 2006 21:58 PST
Search keywords: "vapor pressure calculation"
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