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Molecular Weight of Substances

Definition and molecular weight (molar mass) of some common substances.

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The molecular weight of a substance, also called the molar mass, M, is the mass of 1 mole of that substance, given in M gram. 

In the SI system the unit of M is [kg/kmol] and in the English system the unit is [lb/lbmol], while in the cgs system the unit of M is [g/mol]. Molecular weight is represented by the same number in all unit systems regardless of the system used. For this reason, in many cases the unit for the molecular weight is not mentioned; however, one must realize that it is not a dimensionless parameter.

The molecular weight of a pure compound is determined from its chemical formula and the atomic weights of its elements. The atomic weights of the elements found in organic substances are C = 12.011, H = 1.008, S = 32.065, O = 15.999, and N = 14.007.

Example: The molecular weight of ethanol (C2H5OH)
To calculate the molecular weight of ethanol, the molecular weight of each atom in the molecule is summed:

Methanol = 2*12.011[kg/kmol] + 6*1.008[kg/kmol] + 1*15.999 [kg/kmol] = 46.069 [kg/kmol]

See also Physical data for hydrocarbons, Physical data for alcohols and carboxylic acids, Physical data for organic nitrogen compounds and Physical data for organic sulfur compounds


Molecular Weight

Acetylene, C2H2 26.038
Air 28.966
Ammonia (R-717) 17.02
Argon, Ar 39.948
Benzene 78.114
n - Butane, C4H10 58.124
1,2 - Butadiene 54.092
1-Butene 56.108
cis -2-Butene 56.108
trans-2-Butene 56.108
Butylene 56.06
Carbon Dioxide, CO2 44.01
Carbon Disulphide 76.13
Carbon Monoxide, CO 28.011
Chlorine 70.906
Cyclohexane 84.162
Cyclopentane 70.135
n - Decane 142.286
Deuterium 2.014
2,3 - Dimetylbutane 86.178
2,2 - Dimethylpentane 100.205
Diisobutyl 114.232
Duoderane 170.21
Ethane, C2H6 30.070
Ethene 28.05
Ethyl Alcohol 46.07
Ethylbenzene 106.168
Ethyl Chloride 64.515
3 - Ethylpentane 100.205
Ethylene, C2H4 28.054
Fluorine 37.996
Helium, He 4.002602
n - Heptane 100.205
n - Hexane 86.178
Hydrochloric Acid 36.47
Hydrogen, H2 2.016
Hydrogen Chloride 36.461
Hydrogen Sulfide 34.076
Hydroxyl, OH 17.01
Isobutane (2-Metyl propane) 58.124
Isobutene 56.108
Isooctane 210.63
Isopentane 72.151
Isoprene 68.119
Isopropylbenzene 120.195
Krypton 83.80
Methane, CH4 16.043
Methyl Alcohol 32.04
Methyl Butane 72.15
Methyl Chloride 50.488
Methylcyclohexane 98.189
Methylcyclopentane 84.162
2 - Methylhexane 100.205
2 - Methylpentane 86.178
Natural Gas 19.00
Neon, Ne 20.179
Neohexane 86.178
Neopentane 72.151
Nitric Oxide, NO 30.006
Nitrogen, N2 28.0134
Nitrous Oxide, N2O 44.013
n - Nonane 128.259
n - Octane 114.232
Oxygen, O2 31.9988
Ozone 47.998
n - Pentane 72.151
Pentylene 70.08
Propane, C3H8 44.097
Propene 42.081
Propylene 42.08
R-11 137.37
R-12 120.92
R-22 86.48
R-114 170.93
R-123 152.93
R-134a 102.03
R-611 60.05
Styrene 104.152
Sulfur 32.02
Sulfur Dioxide (Sulphur Dioxide) 64.06
Sulfuric Oxide 48.1
Toluene, toluol 92.141
Triptane 100.205
Xenon 131.30
o - Xylene, xylol 106.168
Water Vapor - Steam, H2O 18.02

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