1011. Solving a chemistry problem

3. Cyanide was added to the sample solution (the sample weight is 0.4080 g) containing Pb, Zn, Mg and Zn was bound. 42.2 ml of 0.0200 M Trilon B were used for the titration of Pb and Mg. Then the Pb - EDTA complex in this solution was destroyed by adding 2,3-dimercaptopropanol. The released EDTA was titrated using 19.3 ml of a 0.0076 M Mg2 solution. Then Zn was unmasked and titrated with 28.6 ml of 0.020 M Trilon B. Determine the mass fraction of Pb, Zn, Mg in the sample.