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 ```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?```
 ```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 20º C, the resulting VP is 21.86 [mmHg] (aproximately 22, as you posted). At 0º C, it would be 6.74 [mmHg]; and at 40º 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" ://www.google.com/search?sourceid=navclient&ie=UTF-8&rls=GGLG,GGLG:2006-06,GGLG:en&q=%22vapor+pressure+calculation%22```