docs: def __update_molar_mass(self, compound='', element=''):
          """Re-calculate the molar mass of the element given due to stoichiometric changes
  
          Parameters:
          ==========
          compound: string (default is '') name of compound
          element: string (default is '') name of element
          """
          _molar_mass_element = 0
          list_ratio = self.stack[compound][element]['isotopes']['isotopic_ratio']
          list_mass = self.stack[compound][element]['isotopes']['mass']['value']
          ratio_mass = zip(list_ratio, list_mass)
          for _ratio, _mass in ratio_mass:
              _molar_mass_element += np.float(_ratio) * np.float(_mass)
          self.stack[compound][element]['molar_mass']['value'] = _molar_mass_element