Question

A 0.4485 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 15.00 mL aliquot of this solution was buffered, and titration of the lead, copper, and zinc in solution required 34.07 mL of 0.001534 M EDTA. Thiosulfate was used to mask the copper in a second 20.00 mL aliquot. Titration of the lead and zinc in this aliquot required 34.56 mL of the EDTA solution. Finally, cyanide was used to mask the copper and the zinc in a third 25.00 mL aliquot. Titration of the lead in this aliquot required 25.76 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.

Cu%=

Zn%=

Pb%=

Sn%=

Answer 1

Pb, Cu, Zn react in a ratio 1:1 with EDTA

Start with lead titration:

0.02576 L x 0.001534 mol/L = 3.952x10^-5 mol Pb in 25 mL aliquot or

3.952x10^-5 mol x250mL/25 mL = 3.952x10^-4 mol Pb in 250 mL (all sample) or

3.952x10^-4 mol x 207.2 g Pb/mol = 0.082 g Pb in sample

Titration of the lead and zinc:

0.03456 L x 0.001534 mol/L = 5.3015 x10^-5 mol Zn+Pb in 20 mL aliquot

In 20 mL aliquot, Pb content is

3.952x10^-5 mol x 20 mL/25mL = 3.1616 x10^-5 mol Pb in 20 mL aliquot

5.3015 x10^-5 mol Zn+Pb   - 3.1616 x10^-5 mol Pb =

            = 2.1399 x10^-5 mol 20 mL Zn in aliquot   or

2.1399 x10^-5 mol x 250 mL/20 mL = 26.748 x10^-5mol Zn in sample or

26.748 x10^-5mol x 65.4 g/mol = 0.0175 g Zn in sample

titration of the lead, copper, and zinc

0.03407 L x 0.001534 mol/L = 5.2263x10^-5 mol Zn+Pb+Cu in 15 mL aliqout

In 15 mL aliquot there are:

2.1399 x10^-5 mol Zn x 15 mL/20 mL = 1.6049 x10^-5mol Zn and

3.952x10^-5 mol Pb x 15 mL / 25 mL =2.3712 x10^-5 mol Pb

5.2263x10^-5 mol - 1.6049 x10^-5mol Zn - 2.3712 x10^-5 mol Pb =

      = 1.2502 x10^-5 mol Cu in aliquot

1.2502 mol x10^-5 Cu x 250mL/15mLx 63.55 g/mol = 0.01324 g Cu in sample

Cu%=100x0.01324 g /0.4485 g = 2.952%

Zn%=100x0.0175 g /0.4485 g = 3.902%

Pb%=100x0.0082 g /0.4485 g = 18.28%

Sn%= what remains to 100%