A 0.4866 g sample of pewter, containing tin, lead, copper, and zinc, was dissolv

Question

A 0.4866 g sample of pewter, containing tin, lead, copper, and zinc, was dissolved in acid. Tin was precipitated as SnO_2 4H_2 O 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 34.67 mL of 0.001523 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 34.64 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 25.38 mL of the EDTA solution. Determine the percent composition by mass of each metal in the pewter sample.

Explanation / Answer

total weight of sample = 0.4866 g

Step 1 : Find total moles of Pb+Cu+Zn

Total initial volume of solution (Pb,Cu,Zn) = 250 ml = 0.250 L

Total EDTA used for titrating (Pb,Cu,Zn) = 34.67 ml of 0.001523 M

moles of EDTA used = molarity x volume = 0.001523 M x 0.03467 L = 5.28 x 10^-5 mols

So, moles of Pb+Cu+Zn = 5.28 x 10^-5 mols in 34.67 ml

Molarity of (Pb+Cu+Zn) in total aliquot volume = moles/L = 5.28 x 10^-5/(0.02 + 0.03467) = 9.66 x 10^-4 M

moles of (Pb+Cu+Zn) in orginal solution = 9.66 x 10^-4 x 0.250 = 2.415 x 10^-4 mols

step 2 : Calculate moles of Pb+Zn in solution

moles of EDTA required for Pb+Zn = 0.001523 M x 0.03464 L = 5.27 x 10^-5 mols

molarity of (Pb+Zn) in aliquot volume = 5.27 x 10^-5/(0.025+0.03464) = 8.84 x 10^-4 M

moles of (Pb+Zn) in original solution = 8.84 x 10^-4 x 0.250 = 2.21 x 10^-4 mols

So, moles of Cu in solution = 2.415 x 10^-4 - 2.21 x 10^-4 = 2.03 x 10^-5 mols

Step 3 : Calculate moles of Pb in solution

moles of EDTA required to titrate Pb = 0.001523 M x 0.02538 L = 3.86 x 10^-5 mols

Molarity of Pb in aliquot volume = 3.68 x 10^-5/(0.030 + 0.02538) = 6.98 x 10^-4 M

moles of Pb in original solution = 6.98 x 10^-4 x 0.250 = 1.74 x 10^-4 mols

So we have now,

Step 4 : Total moles of each

moles of Cu = 2.05 x 10^-5 mols

moles of Pb = 1.74 x 10^-4 mols

moles of Zn = 2.21 x 10^-4 - 1.74 x 10^-4 = 4.70 x 10^-5 mols

Step 5 : convert moles to mass (mass = moles x molar mass)

mass of Cu = 2.05 x 10^-5 x 63.546 = 1.303 x 10^-3 g

mass of Pb = 1.74 x 10^-4 x 207.2 = 0.036 g

mass of Zn = 4.70 x 10^-5 x 65.38 = 3.073 x 10^-3 g

total mass (Pb+Cu+Zn) = 0.0404 g

mass of Sn = 0.4866 - 0.0404 = 0.4462 g

step 6 : Calculate mass % of each metal

mass % of Cu = 100 x 1.303 x 10^-3/0.4866 = 0.27%

mass % of Pb = 100 x 0.036/0.4866 = 7.40%

mass % of Zn = 100 x 3.073 x 10^-3/0.4866 = 0.63%

mass % of Sn = 100 x 0.4462/0.4866 = 91.70%

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