Home >> Science & Nature >> Science

How to Determine Water Vapor Pressure From a Table Chart

When you heat water, the molecules begin moving quickly and with more energy. Some of the molecules in the heated water will have enough energy to escape from the surface of the liquid into the air. At the same time, water molecules already in the air will meet the surface and incorporate themselves in the water again. When this happens in a closed container, the molecules in the air above the heated water create vapor pressure. Scientists express this pressure in millimeters of mercury.

Things You'll Need

Pencil
Paper
Calculator
Internet access or hard-copy table of water vapor pressure

Instructions

- 1
  Write the temperature for which you want to determine water vapor pressure. For example, write "63.14 degrees Fahrenheit."
- 2
  Subtract 32 from the number you've written. Multiply your answer by 5/9 using a calculator. These actions will convert your temperature in degrees Fahrenheit to degrees Celsius. For example, (63.14 - 32) x 5/9 = 17.3 degrees Celsius.
- 3
  Consult a hard-copy or online water vapor pressure table. For example, access the Volunteer State Community College site for "Vapor Pressure of Water (mm Hg) vs. Temperature (degrees C)."
- 4
  Find the number that is left of the decimal point of your calculated temperature in degrees Celsius in the far left-hand column of the table. For example, the number to the left of the decimal point of your calculated temperature is 17, which is halfway down the far left-hand column of the Volunteer State Community College table.
- 5
  Read left-to-right across the row of the table beginning with the number you have identified in the left-hand column. Find the column that has, at the top of it, the number to the right of the decimal point of your calculated temperature in degrees Celsius. For example, the number to the right of the decimal point of your calculated temperature is 3. Read left-to-right across the table row beginning with the number 17 until you reach the column that has 3 (0.3) at the top.
- 6
  Read the number in the box at the intersection between the horizontal row and vertical column you have identified. Finishing the example, you will find "14.809" in the box. The water vapor pressure of water at 17.3 degrees Celsius, or 63.14 degrees Fahrenheit, is 14.809 millimeters of mercury (mm Hg).

Science