How to Calculate Saturated Water Vapor Pressure

Saturated water vapor pressure is the measure of the amount of evaporated water inside a closed vessel that contains liquid water and air. Inside the vessel, the number of water molecules evaporating is equal to the number of water molecules condensing back into the liquid. The Antoine equation governs the measure of water vapor pressure. The equation was is a relationship chemists derived from experimental data. The expression relates the pressure of water vapor to its temperature in terms of three constants (A, B and C) that are valid for a given temperature range.

Instructions

1




A common Celsius thermometer

Measure the temperature of the air in Celsius with a thermometer. For example, say the temperature is 70 degrees Celsius.

2

Convert the temperature to degrees Kelvin. Add 273.15 to your 70 degrees (or any other reading), which gives you a temperature of 343.15 Kelvin.

3

Find the appropriate values of the Antoine constants A, B and C for the temperature at the National Institute of Standards and Technology's online Chemistry WebBook. For a temperature of 343.15 K, the constants are A = 5.08354, B = 1663.125, and C = -45.622.

4

Calculate the value of the vapor pressure in pascals utilizing the Antoine equation. The equation is:

P = 10^ ( A - ( B / (C+T) ) ),

where "T" is the temperature and "P" is the pressure. For the example problem, you have:

P = 10^( 5.08354 - ( 1663.125 / ( -45.622 + 343.15 ) )

= 10^( 5.08354 - ( 1663.125 / ( 297.53 )

= 10^( 5.08354 - 5.59 )

= 10^( -0.51 )

= 0.31 pascals.

The water vapor pressure is 0.31 pascals.