How to calculate vapor pressure

Written by lynn keller
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How to calculate vapor pressure
(pressure-gauge image by Dusan Radivojevic from Fotolia.com)

To estimate the vapour pressure of a liquid or a solid a moderately easy formula to use is the Antoine Equation. The Antoine Equation is based upon experimental vapour pressures measured over a set temperature range.

Skill level:
Moderate

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Things you need

  • Calculator with LOG functions
  • Temperature (in Kelvin) of chosen liquid or solid
  • Antoine Equation coefficients for the chosen liquid or solid

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Instructions

  1. 1

    Determine the temperature of the chosen liquid or solid. Convert this temperature to Kelvin if it is not in degrees Kelvin. The conversions are listed below.

  2. 2

    Find the Antoine Equation values for the chosen liquid or solid. References to find these values are listed below. Log (P) = A ' (B / (T + C)) P = vapour pressure (bar) T = temperature (K)

  3. 3

    Plug in the values into the equation. Log (P) = A ' (B / (T + C)) P = vapour pressure (bar) T = temperature (K)

  4. 4

    Calculate the log of the solution.

  5. 5

    For example, if methane was the chosen material and T = 150 degrees Kelvin. From the National Institute of Standards and Technology (NIST) Chemistry Webbook the Antoine Coefficients for methane are as follows: Temperature (K) = 90.99 - 189.99 A = 3.98950 B= 443.028 C=-0.49 Reference: Prydz and Goodwin, 1972

    Coefficents calculated by NIST from author's data.

  6. 6

    Using the formula, Log (P) = A -- (B / (T+C)) plug in the numbers from above: Log (P) = 3.98950 -- (443.028 / (150K - .049)) Log (P) = 1.03501 P = 10.8396 bar (Use the inverse of the log function to get this number)

Tips and warnings

  • To convert Celsius to Kelvin add 273.15.
  • To convert Fahrenheit to Kelvin add 459.67 and multiply the sum by 5/9.
  • To convert Rankin to Kelvin multiply by 5/9.
  • A good source of Antoine Equation coefficients is the National Institute of Standards and Technology (NIST) Chemistry Webbook.
  • Vapour pressure is dependent only on temperature.

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