Mole Weight Calculation
If you need to calculate the mole weight of a gas or liquid mixture, the calculation isn't very difficult. It's a great application for a spreadsheet, because the calculation is the repetitive multiplication of the mole weight of each component in the analysis by its mole fraction, and then the summation of all these products.
The analysis should be normalized before the mole weight calculation is done. Normalization is the process of dividing each component concentration by the total of the components so that they do add up to 1.0000 when the normalization is complete, even if they did not quite add up beforehand. This may seem like a fudge factor sort of operation, but in fact it is very common, particularly when the analysis is provided in mole percentage rather than mole fraction. Even if the analysis components add up to 100.00%, the digits have to be shifted over by two to make them mole fractions before the math is done. This counts as a normalization operation, albeit a trivial one.
Species Mole Weights and Other Physical Properties
The question may occur to you that a bunch of physical constants will be required in order to perform the calculation. This is true, and there are more sources for these constants than you can shake a stick at. However, some of the major ones are listed below. It's always good to be able to say where you got any constant used in a calculation, otherwise it's just a "magic" number nobody can explain later on. Sticking to well-known and recognized published standards is a good way to keep people from turning their nose up at your work.
GPA 2145, Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas Industry, 2009
ISO 6976, Natural gas -- Calculation of calorific values, density, relative density and Wobbe index from composition, 1997-12-15
ASTM D3588-98, Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density of Gaseous Fuels, 2003
The most appropriate one for you to choose may depend on your application, your industry or your location. Europeans should think carefully about using anything other than the ISO standard. North Americans and people in the petroleum industry generally should otherwise gravitate toward standards published by AGA or GPA unless there is a good reason to do otherwise. If you have free reign to go hunting for any of the above that please you, I would recommend the ISO 6976 publication because the accompanying documentation is the best IMHO.
Calculation to follow.