How to calculate the vapor pressure

Estimating the vapor pressure of a liquid or a solid, a moderately easy-to-use formula is the Antoine equation. The Antoine equation describes the relationship between vapor saturation pressure and the temperature of pure substances.

You will need to:
  • Calculator with register of functions
  • Temperature (in Kelvin) of liquids or solids chosen
  • Coefficients of the Antoine equation for the liquid or solid chosen
Steps to follow:

one

Determine the temperature of the chosen liquid or solid. Convert this temperature to Kelvin, in Spain we use the Celsius, so you should look at conversion tables. The conversions are listed below.

two

Find the values ​​of the Antoine equation for the chosen liquid or solid. The references to find these values ​​are shown below.

The formula is the following: Log (P) = A '(B / (T + C))

  • P = vapor pressure (Pa)
  • T = temperature (K)

3

Join the values ​​in the equation. Log (P) = A '(B / (T + C))

  • P = vapor pressure (Pa)
  • T = temperature (K)

4

Calculate the solution using the previous equation.

5

For example, if ethanol was the chosen material and T = 351.47 Kelvin. The Antoine Coefficients for methane are the following:

  • A = 10.32907
  • B = 1642.89
  • C = - 42.85

6

Using the formula, Log (P) = A - (B / (T + C)) connect the numbers above:

  • Log (P) = 10.32907 - (1642.89 / (351.47 K - 42.85))
  • Log (P) = 5, 005727
  • P = 101327, 511 Pa

(Use the inverse of the registration function to get this number).

7

See this article to learn how to calculate wattage.

Tips
  • To convert Celsius degrees to Kelvin add 273.15.
  • To convert Fahrenheit to Kelvin add 459.67 and multiply the result by 5.9.
  • To convert Rankin to Kelvin multiply by 9.5.
  • A good source of coefficients of the Antoine equation is the National Institute of Standards and Technology (NIST) Chemistry Webbook.
  • The vapor pressure depends only on the temperature.