@property<EOL><INDENT>def mass_fractions(self):<DEDENT>
if self.__mass_fractions:<EOL><INDENT>return self.__mass_fractions<EOL><DEDENT>else:<EOL><INDENT>self.__mass_fractions =  mass_fractions(self.atoms, self.MW)<EOL>return self.__mass_fractions<EOL><DEDENT>
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}