# How to calculate vapor pressure

Written by lynn keller
• Share
• Tweet
• Share
• Pin
• Email

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

### Things you need

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

## 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.

### Don't Miss

• All types
• Articles
• Slideshows
• Videos
##### Sort:
• Most relevant
• Most popular
• Most recent

By using the eHow.co.uk site, you consent to the use of cookies. For more information, please see our Cookie policy.