code: def _calc_a(self, y_C, y_H, y_O, y_N, y_S):
          
  
          return 1 / (y_C/mm("C") + y_H/mm("H") + y_O/mm("O") + y_N/mm("N") +
                      y_S/mm("S"))
docs: Calculate the mean atomic weight for the specified element mass
          fractions.
          
          :param y_C: Carbon mass fraction
          :param y_H: Hydrogen mass fraction
          :param y_O: Oxygen mass fraction
          :param y_N: Nitrogen mass fraction
          :param y_S: Sulphur mass fraction
         
          :returns: [kg/kmol] mean atomic weight
  
          See equation at bottom of page 538 of Merrick1983a.