Question

A 0.4598 g sample of pewter, containing tin, lead, copper, and zinc, was dissolved in acid. Tin was precipitated as SnO2·4H2O and removed by filtration. The resulting filtrate and washings were diluted to a total volume of 250.0 mL. A 20.00 mL aliquot of this solution was buffered, and titration of the lead, copper, and zinc in solution required 34.81 mL of 0.001487 M EDTA. Thiosulfate was used to mask the copper in a second 25.00 mL aliquot. Titration of the lead and zinc in this aliquot required 34.79 mL of the EDTA solution. Finally, cyanide was used to mask the copper and the zinc in a third 30.00 mL aliquot. Titration of the lead in this aliquot required 24.40 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.

Answer 1

Cu*2, Znt2 and Pbt2 forms 1:1 complex with EDTA Cut2 EDTA[Cu(EDTA)]2 Zn*2+ EDTAIZn(EDTA))2 Pb+2 + EDTA-4 > [Pb(EDTA)]-2 Molarity of EDTA 0.001487 M 1) For the 30 mL third aliquot which contains on Lead, the volume of EDTA required = 24.40 mL Number of moles of EDTA = M*V = 0.001487 M*24.40 mL = 0.0362828 mmol Number of moles of Pb+2-no, of moles of EDTA-0.0362828 mmol Mass of Pb*2 in 30 mL moles*mol.wt. 0.0362828 mmol*207.2 g/mol- 7.517796 mg Mass of Pbt2 in 250 mL of aliquot 7.849888 mg* 250 mL/30 mL 62.65 mg 2) For the 25 mL second aliquot which contains on Lead and Zinc, the volume of EDTA required-34.79 mL Number of moles of EDTA = M*V = 0.001487 M* 34.79 mL = 0.05173273 mmol Number of moles of Zn+2 and Pb+2 in 25 mL = no, of moles of EDTA = 0.05173273 mmol Number of moles of Pb+2 in 30 mL 0.0362828 mmol Number of moles of Pb+2 in 25 mL = 0.0362828 mmol *25/30 0.030235667 mmol Number of moles of Zn+2-0.05173273-0.030235667 = 0.02149706 mmol Mass of Zn+2 in 25 mL = 0.02149706 mmol* 65.38 g/mol = 1.405478 mg Mass of Zn+2 in 250 mL = 14.05 mg

3) For the 20 mL first aliquot which contains on Copper, Lead and Zinc, the volume of EDTA required = 34.81 mL Number of moles of EDTA M*V 0.001487 M* 34.81 mL 0.05176247 mmol Number of moles of Cu*2, Znt2 and Pb*2- no. of moles of EDTA 0.05176247 mmol Number of moles of Znt2 and Pbt2 in 25 mL0.05173273 mmol Number of moles of Znt2 and Pb*2 in 20 mL 0.05173273 mmol*20/25 0.04138618 mmol Number of moles of Cut20.05176247 0.04138618 0.01037629 mmol Mass of Cu*2 in 20 mL of aliquot 0.01037629 mmol*63.55 g/mol 0.659413 mg Mass of Cut2 in 250mL of aliquot 0.659413 *250/20 8.24 mg Mass of Pb 62.65 mg-0.06265 g Mass of Zn = 14.05 mg 0.01405 g Mass of Cu 8.24 mg 0.00824 g Mass of Sn 0.4598-0.06265 0.01405 0.00824 0.37486 g Zotal x100 0.00824 Total x100 0.0140s Total x100-006265 x1 00-1 .7990 0.4598 Mass ofx100 04598 %Zn x1 00 3.06% ass of Pp x 1 00 13.63% ass of Snx 100 0,4598 %Sn =-- x1 00 81 .53%