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Engineering ToolBox > The Individual and Universal Gas Constant
The Individual and the Universal Gas Constant are known from the Ideal Gas Law.
The Individual Gas Constant depends on the particular gas and is related to the molecular weight of the gas. The value is independent of temperature.
In the imperial system the units for the individual gas constant are ft.lb/slug.oR. In the SI system the units are J/kg.K.
The Individual Gas Constant for some common gases are listed below:
| Gas | Individual Gas Constant - R | ||
| Imperial Units (ft lb/slug oR) | SI Units (J/kg K) | Molecular Weight (kg/kgmole) | |
| Argon, Ar | 208 | 39.94 | |
| Carbon Dioxide, CO2 | 1,130 | 188.9 | 44.01 |
| Carbon Monoxide, CO | 297 | 28.01 | |
| Helium, He | 12,420 | 2,077 | 4.003 |
| Hydrogen, H2 | 24,660 | 4,124 | 2.016 |
| Methane - natural gas, CH4 | 3,099 | 518.3 | 16.04 |
| Nitrogen, N2 | 1,775 | 296.8 | 28.02 |
| Oxygen, O2 | 1,554 | 259.8 | 32 |
| Propane, C3H8 | 189 | 44.09 | |
| Sulfur dioxide, SO2 | 130 | 64.07 | |
| Air | 1,716 | 286.9 | 28.97 |
| Water vapor | 2,760 | 461.5 | 18.02 |
The Universal Gas Constant - Ru - appears in the ideal gas law and can be expressed as the product between the Individual Gas Constant - R - for the particular gas - and the Molecular Weight - Mgas - for the gas, and is the same for all ideal or perfect gases:
Ru = Mgas R (1)
where
Ru = universal gas constant
Mgas = molecular weight of the ideal gas or mixture of gases
The molar weight of a gas can be calculated like
Mgas = (f1/M1 + .... + fn/Mn) (2)
where
f = mass of the gas relative to the total mass of the mixture
M = molecular weight of the gas
The universal gas constant cab be defined in terms of Boltzmann's constant k as:
Ru = k NA (2)
where
k = Boltzmann's constant = 1,3807 x 10-23 (J/K)
NA = Avagadros Number = 6,022 x 1023 (1/mol)