At any temperature, a liquid will partly exist in a gaseous state because it is constantly subject to evaporation. Vapor pressure is the pressure of the gaseous phase of the liquid in a closed container at equilibrium. In this equilibrium state, some of the liquid is constantly evaporating to gas, as well as condensing back to liquid at a constant rate. If you know the vapor pressure, you can also calculate the concentration of the gaseous phase in parts per million (ppm).
Divide the vapor pressure of the gas by the total pressure of the gas and liquid combined. The result is known as the mole fraction of the gas and is a measure of gas concentration. This step assumes that both the vapor pressure and total pressure are known quantities.
Multiply by the mole fraction, calculated above, by 100 to get the gas volume as a percentage of the total volume.
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Multiply the percent volume, calculated above, by 10,000 to get the gas concentration in parts per million (ppm).