Question

A 0.4775 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 200.0 mL. A 20.00 mL aliquot of this solution was buffered, and titration of the lead, copper, and zinc in solution required 35.98 mL of 0.001455 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 34.00 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 27.43 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.

Answer 1

Sample weight = 0.4775 g

volume of EDTA used for Pb titration = 27.43 ml

moles of EDTA = molarity x volume = 0.001455 x 0.02743 = 3.99 x 10^-5 mols

moles of Pb = 4 x 10^-5 mols

Molarity of Pb = 4 x 10^-5/(0.030 + 0.02743) = 6.95 x 10^-4 M

moles of Pb in total solution = 6.95 x 10^-4 x 0.2 = 1.39 x 10^-4 mols

mass of Pb = 1.39 x 10^-4 x 207.2 = 0.029 g

Volume of EDTA used to titrate Pb and Zn = 0.001455 x 0.034 = 4.95 x 10^-5 mols

moles of Pb and Zn = 5 x 10^-5 mols

Molarity of Pb and Zn = 5 x 10^-5/(0.025 + 0.034) = 8.38 x 10^-4 M

moles of Pb and Zn in total volume = 8.38 x 10^-4 x 0.2 = 1.68 x 10^-4 mols

moles of Zn = 1.68 x 10^-4 - 1.39 x 10^-4 = 2.87 x 10^-5 moles

mass od Zn in sample = 2.87 x 10^-5 x 65.38 = 0.0019 g

Volume of EDTA used to titrate Pb,Zn,Cu = 0.001455 x 0.03598 = 5.24 x 10^-5 mols

moles of Pb,Zn,Cu = 5.24 x 10^-5 mols

Molarity of Pb,Zn,Cu = 5.24 x 10^-5/(0.2 + 0.03598) = 9.352 x 10^-4 M

moles of Pb,Zn,Cu in total volume = 9.352 x 10^-4 x 0.2 = 1.9 x 10^-4 mols

moles of Cu = 1.9 x 10^-4 - (1.39 x 10^-4 + 2.87 x 10^-5) = 2.23 x 10^-5 mols

mass of Cu = 2.23 x 10^-5 x 63.546 = 0.0014 g

Mass of Sn = 0.4775 - (0.029 + 0.0019 + 0.0014) = 0.4452 g

Percent composition of metals would be thus,

Sn = (0.4452/0.4775) x 100 = 93.23%

Cu = (0.0014/0.4775) x 100 = 0.30%

Zn = (0.0019/0.4775) x 100 = 0.40%

Pb = (0.029/0.4775) x 100 = 6.07%