For most calculations you will only need to complete the above information as the mg/m3 concentration is understood to be referenced at 25ºC and 1atm (1013.25mbar)
However if you need to consider different conditions of pressure and temperature, you will need to introduce the following information
ppm: parts per million. Concentration by volume of one part of gas per million parts of air. This definition is only valid for gases. For liquids or solids this calculator cannot be used.
mg/m3: milligrams of contaminant per cubit meter. Usually it is referenced at a pressure of 1atm and 25ºC.
Molecular weight: Molecular weight is a measure of the sum of the atomic weights of the atoms in a molecule.So if you don´t know the molecular weight of a gas but you know you can calculate easily the molecular weight using the periodic table (e.g.NIST periodic table)For example the molecular weight of H2S is 2x1.008 + 32.06 = 34.08
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