docs: def get_total_mass(self):
          """Returns the total mass in g/mol.
  
          Args:
              None
  
          Returns:
              float:
          """
          try:
              mass = self.loc[:, 'mass'].sum()
          except KeyError:
              mass_molecule = self.add_data('mass')
              mass = mass_molecule.loc[:, 'mass'].sum()
          return mass