code: def mass_fractions(self):
          r'''Dictionary of atom:mass-weighted fractional occurence of elements.
          Useful when performing mass balances. For atom-fraction occurences, see
          :obj:`atom_fractions`.
  
          Examples
          --------
          >>> Chemical('water').mass_fractions
          {'H': 0.11189834407236524, 'O': 0.8881016559276347}
          '''
          if self.__mass_fractions:
              return self.__mass_fractions
          else:
              self.__mass_fractions =  mass_fractions(self.atoms, self.MW)
              return self.__mass_fractionf mass_fractions(self):
          r'''Dictionary of atom:mass-weighted fractional occurence of elements.
          Useful when performing mass balances. For atom-fraction occurences, see
          :obj:`atom_fractions`.
  
          Examples
          --------
          >>> Chemical('water').mass_fractions
          {'H': 0.11189834407236524, 'O': 0.8881016559276347}
          '''
          if self.__mass_fractions:
              return self.__mass_fractions
          else:
              self.__mass_fractions =  mass_fractions(self.atoms, self.MW)
              return self.__mass_fractions
docs: r'''Dictionary of atom:mass-weighted fractional occurence of elements.
          Useful when performing mass balances. For atom-fraction occurences, see
          :obj:`atom_fractions`.
  
          Examples
          --------
          >>> Chemical('water').mass_fractions
          {'H': 0.11189834407236524, 'O': 0.8881016559276347}