Elements

The MW of O is                15.9995 kg/kmol, and of O2 is 31.999 kg/kmol

The MW of N is                14.0065 kg/kmol, and of N2 is 28.013 kg/kmol

The MW of H is                1.0080 kg/kmol, and of H2 is 2.0160 kg/kmol

Carbon     C is                   12.011 kg/kmol

Sulphur    S is                     32.064

Fluorine    F is                    18.998

Chlorine Cl is                      35.457

Argon    Ar is                     39.948

Compounds

So Water/steam            H2O is               18.0155 kg/kmol

Carb. Monoxide            CO is                 28.0105

Carb. Dioxide                CO2 is               44.010

Sulphur Dioxide              SO2 is               64.063

Methane                          CH4 is              16.043

Ethane                             C2H6 is            30.070

Propane                           C3H8 is            44.097

Ethylene                          C2H4 is            28.054

Butane                            C4H10 is           58.124

AIR                                             is            28.96 kg/kmol but in volume calculations use 28.85 kg/kmol.