Densities, molecular weight and chemical formulas of some common gases can be found in the table

Densities, molecular weight and chemical formulas of some common gases can be found in the table below:

¹⁾ NTP - Normal Temperature and Pressure - is defined as 20^oC (293.15 K, 68^oF) and 1 atm ( 101.325 kN/m², 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr)

²⁾ STP - Standard Temperature and Pressure - is defined as  0^oC (273.15 K, 32^oF) and 1 atm (101.325 kN/m², 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr)

• 1 lb_m/ft³ = 16.018 kg/m³
• 1 kg/m³ = 0.0624 lb_m/ft³

Note that even if pounds per cubic foot is often used as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can divide pounds per cubic foot by 32.2 for a rough value in slugs.

Gas	Formula	Molecular weight	Density - ρ -
			(kg/m3)	(lb/ft3)
Acetylene (ethyne)	C2H2	26	1.0921)1.1702)	0.06821) 0.07292)
Air		29	1.2051) 1.2932)	0.07521)0.08062)
Ammonia	NH3	17.031	0.7171) 0.7692)	0.04481) 0.04802)
Argon	Ar	39.948	1.6611)1.78372)	0.10371)0.1113532)
Benzene	C6H6	78.11	3.486	0.20643
Blast furnace gas			1.2502)	0.07802)
Butane	C4H10	58.1	2.4891)2.52)	0.15541) 0.1562)
Butylene (Butene)	C4H8	56.11	2.504	0.1482)
Carbon dioxide	CO2	44.01	1.8421)1.9772)	0.11501)0.12342)
Carbon disulphide		76.13
Carbon monoxide	CO	28.01	1.1651)1.2502)	0.07271) 0.07802)
Carbureted Water Gas				0.048
Chlorine	Cl2	70.906	2.9941)	0.18691)
Coal gas			0.582)
Coke Oven Gas				0.0342)
Combustion products			1.112)	0.0692)
Cyclohexane		84.16
Digester Gas (Sewage or Biogas)				0.062
Ethane	C2H6	30.07	1.2641)	0.07891)
Ethyl Alcohol		46.07
Ethyl Chloride		64.52
Ethylene	C2H4	28.03	1.2602)	0.07862)
Helium	He	4.02	0.16641)0.17852)	0.010391)0.0111432)
N-Heptane		100.20
Hexane		86.17
Hydrogen	H2	2.016	0.08992)	0.00562)
Hydrochloric Acid		36.47	1.632)
Hydrogen Chloride	HCl	36.5	1.5281)	0.09541)
Hydrogen Sulfide	H2S	34.076	1.4341)	0.08951)
Krypton			3.742)
Methane	CH4	16.043	0.6681)0.7172)	0.04171) 0.04472)
Methyl Alcohol		32.04
Methyl Butane		72.15
Methyl Chloride		50.49
Natural gas		19.5	0.7 - 0.92)	0.044 - 0.0562)
Neon	Ne	20.179	0.89992)	0.0561792)
Nitric oxide	NO	30.0	1.2491)	0.07801)
Nitrogen	N2	28.02	1.1651)1.25062)	0.07271) 0.0780722)
Nitrogen Dioxide	NO2	46.006
N-Octane		114.22
Nitrous Oxide	N2O	44.013		0.1141)
Nitrous Trioxide	NO3	62.005
Oxygen	O2	32	1.3311) 1.42902)	0.08311) 0.0892102)
Ozone	O3	48.0	2.142)	0.125
N-Pentane		72.15
Iso-Pentane		72.15
Propane	C3H8	44.09	1.8821)	0.11751)
Propene (propylene)	C3H6	42.1	1.7481)	0.10911)
R-11		137.37
R-12		120.92
R-22		86.48
R-114		170.93
R-123		152.93
R-134a		102.03
Sasol				0.032
Sulfur	S	32.06		0.135
Sulfur Dioxide	SO2	64.06	2.2791) 2.9262)	0.17031) 0.18282)
Sulfur Trioxide	SO3	80.062
Sulfuric Oxide	SO	48.063
Toluene	C7H8	92.141	4.111	0.2435
Water Vapor, steam	H2O	18.016	0.804	0.048
Water gas (bituminous)				0.054
Xenon			5.862)