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ISO TR 29922-2017 provides a definition for standard dry air which specifies an air molar mass of 28,965 46 ± 0,000 17 kg·kmol-1. [2] GPA 2145:2009 is published by the Gas Processors Association. It provides a molar mass for air of 28.9625 g/mol, and provides a composition for standard dry air as a footnote. [3]
R ∗ = 8.314 32 × 10 3 N⋅m⋅kmol −1 ⋅K −1 = 8.314 32 J⋅K −1 ⋅mol −1. Note the use of the kilomole, with the resulting factor of 1000 in the constant. The USSA1976 acknowledges that this value is not consistent with the cited values for the Avogadro constant and the Boltzmann constant. [ 13 ]
Molecular weight (M.W.) (for molecular compounds) and formula weight (F.W.) (for non-molecular compounds), are older terms for what is now more correctly called the relative molar mass (M r). [8] This is a dimensionless quantity (i.e., a pure number, without units) equal to the molar mass divided by the molar mass constant .
is the molecular mass of dry air, approximately 4.81 × 10 −26 in kg. [note 1], the specific gas constant for dry air, which using the values presented above would be approximately 287.050 0676 in J⋅kg −1 ⋅K −1. [note 1] Therefore:
The SI value of the mole was chosen on the basis of the historical definition of the mole as the amount of substance that corresponds to the number of atoms in 12 grams of 12 C, [1] which made the mass of a mole of a compound expressed in grams, numerically equal to the average molecular mass or formula mass of the compound expressed in daltons.
= air pollutant concentration, in parts per million by volume mg/m 3 = milligrams of pollutant per cubic meter of air = atmospheric temperature in kelvins = 273.15 + °C 0.08205 = Universal Gas Law constant in atm·l/(mol·K) = molecular weight of the air pollutant (dimensionless)
The molar mass constant, usually denoted by M u, is a physical constant defined as one twelfth of the molar mass of carbon-12: M u = M(12 C)/12. [1] The molar mass of an element or compound is its relative atomic mass (atomic weight) or relative molecular mass (molecular weight or formula weight) multiplied by the molar mass constant.
The average molecular weight of dry air, which can be used to calculate densities or to convert between mole fraction and mass fraction, is about 28.946 [11] or 28.964 [12] [5] g/mol. This is decreased when the air is humid. The relative concentration of gases remains constant until about 10,000 m (33,000 ft). [13]