A 0.4837 g sample of pewter, containing tin, lead, copper, and zinc, was dissolv
ID: 496960 • Letter: A
Question
A 0.4837 g sample of pewter, containing tin, lead, copper, and zinc, was dissolved in acid. Tin was precipitated as SnO_2 4H_2O 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 34.39 mL of 0.001461 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 33.14 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 27.67 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.Explanation / Answer
Metal ion + ETDA --------> Metal - EDTA
1mole 1 mole. 1 mole
0.001461M = 0.001461 mole in 1 litre
= 0.000001461 mole in 1ml
First titration
aliquot = 15 ml
Volume of EDTA consumed = 34.39 ml
Mole of EDTA = 34.39 × 0.000001461 = 0.000050243 mole
Mole of Metal ions(Pb , Cu and Zn) = 0.000050243 mole
Titration 2
aliquot = 20 ml
Volume of EDTA consumed =33.14 ml
Mole of EDTA consumed = 33.14 × 0.000001461= 0.000048417mole
Mole of Metal ions (Zn and Pb) = 0.000048417 mole
Mole of Zn and Pb in 15 ml = (0.048417/20)×15 =0.000036313mole
Titration 3
aliquot = 25 ml
Volume of EDTA consumed = 27.67ml
Mole of EDTA consumed = 27.67 × 0.000001461 =0.000040426mole
Mole of Pb = 0.000040426
Mole of Pb in 15 ml = (0.040426/25)×15 = 0.000024256
Therefore, No of mole of Pb in 15ml= 0.000024256
No of mole of Zn in 15ml = 0.000036313-0.000024256= 0.000012057
No of mole of Cu in 15 ml = 0.000050243-0.000036313 = 0.00001393
No of Mole of Pb in 250ml =(0.000024256/15)×250= 0.00040427
No of mole of Zn in 250 ml =(0.00012057/15)×250=0.00020095
No of mole of Cu in 250ml = (0.00001393/15)×250=0.00023217
Mass of Pb = molar mass × No of mole
= 207.2 × 0.00040427 = 0.08376g
of Pb = (0.08376/0.4837)×100 =17.32
% of Zn = 2.72
% of Cu = 3.05
of Sn = 76.91
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