The Individual and Universal Gas Constant

The Individual and Universal Gas Constant in fluid mechanics and thermodynamics

The Individual and the Universal Gas Constant are known from the Ideal Gas Law.

The Individual Gas Constant - R

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
(g/mole)
Argon, Ar 208 39.94
Acetylene 319 26
Ammonia 488 17
Carbon Dioxide, CO2 1130 188.9 44.01
Carbon Monoxide, CO 297 28.01
Carbonic acid 189 44
Helium, He 12420 2077 4.003
Hydrogen, H2 24660 4124 2.016
Methane - natural gas, CH4 3099 518.3 16.04
Nitrogen, N2 1775 296.8 28.02
Oxygen, O2 1554 259.8 32
Propane, C3H8 189 44.09
Sulfur dioxide, SO2 (sulfuric acid) 130 64.07
Air 1716 286.9 28.97
Water vapor 2760 461.5 18.02

The Universal Gas Constant - Ru

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

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 - Ru - in alternative Units

  • atm.cm3/(mol.K) : 82.0575
  • atm.ft3/(lbmol.K) : 1.31443
  • atm.ft3/(lbmol.oR) : 0.73024
  • atm.l/(mol.K) : 0.08206
  • bar.cm3/(mol.K) : 83.14472
  • bar.l/(mol.K) : 0.08314472
  • Btu/(lbmol.oR) : 1.9859
  • cal/(mol.K) : 1.9859
  • erg/(mol.K) : 83144720
  • hp.h/(lbmol.oR) : 0.0007805
  • inHg.ft3/(lbmol.oR) :  21.85
  • J/(mol.K) :  8.314462 
  • kJ/(kmol.K) :  8.314462
  • J/(kmol.K) : 8314.472
  • (kgf/cm2).l/(mol.K) : 0.084784
  • kPa.cm3/(mol.K) : 8314.472
  • kWh/(lbmol.oR) : 0.000582
  • lbf.ft/(lbmol.oR) : 1545.349
  • mmHg.ft3/(lbmol.K) : 999
  • mmHg.ft3/(lbmol.oR) : 555
  • mmHg.l/(mol.K) : 62.364
  • Pa.m3/(mol.K) : 8.314472
  • psf.ft3/(lbmol.oR) : 1545.349
  • psi.ft3/(lbmol.oR) : 10.73
  • Torr.cm3/(mol.K) : 62364

The Universal Constant defined in Terms of the Boltzmann's Constant

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 = Avogadro Number = 6,022 x 1023 (1/mol)

Related Topics

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