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   Computes the molecular weight of a gas mixture.
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   Form:
   [mM, nM] = MolWt( m, n )
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   Inputs
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   m             (:)     Molecular weight of the constituents
   n             (:)     Number of moles of each constituent

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   Outputs
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   mM            (1,1)   Molecular weight of the mixture
   nM            (1,1)   Number of moles in the mixture

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