A 0.4423 g sample of pewter, containing tin, lead, copper, and zinc, was dissolv
ID: 898479 • Letter: A
Question
A 0.4423 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 300.0 mL. A 15.00 mL aliquot of this solution was buffered, and titration of the lead, copper, and zinc in solution required 34.24 mL of 0.001479 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 33.86 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 27.84 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.
Explanation / Answer
Determination of concentration of Lead
M1V1 = M2V2 where M1 and V1 is the molarity and volume of the pewter sample solution. M2 and V2 is the molarity and volume required for EDTA respectively
25 * M1 = 27.84 * 0.001479
M 1 = 1.65 *10-3 M
Total volume of the solution= 300mL
moles of lead present in 300mL = 1.65 *10-3 *300/1000 = 4.95 * 10-4 mole = 0.103 g
amount of lead in the sample = 0.103 g
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Determination of Zn
Lead + zinc requires 33.86 mL EDTA
apply the above formula
M1V1 = M2V2
M1 = 33.86 mL * 0.001479 M/ 20 mL = 2.5 * 10-3 M
moles of lead + Zinc present in the 300 mL solution = 2.5 * 10-3 * 300/1000 = 7.5 *10-4
moles of lead present in 300mL = 4.95 * 10-4
moles of Zinc present = (7.5-4.95) * 10-4 = 2.55 *10-4
weight of Zinc present = 2.55 *10-4 mol * 65.38 g/mol = 0.0166 g
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Determination of Copper
Lead + zinc + copper requires 34.24 mL EDTA
apply the above formula
M1V1 = M2V2
M1 = 34.24 mL * 0.001479 M/ 15 mL = 3.37 * 10-3 M
moles of lead + Zinc present in the 300 mL solution = 3.37 * 10-3 * 300/1000 = 10.11 *10-4
moles of lead present in 300mL = 4.95 * 10-4
moles of Zinc present = 2.55 *10-4
moles of Copper present = [10.11 - (4.95 +2.55)] *10-4 = 2.61 *10-4 moles
weight of the copper = 2.61 *10-4 moles * 63.54 g/ mole = 0.1658g
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Determination of the weight of Tin
weight of Tin = 0.4423g-weight of (lead + zinc + Tin)
= 0.4423g - 0.2854g
= 0.1569g
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