Question

A 0.3304-g sample of brass (containing lead, zinc, copper, and tin) was dissolved in nitric acid. The sparingly soluble SnO2.4H2O was removed by filtration, and the combined filtrate and washings were then diluted to 500.0 mL. A 10.00-mL aliquot was suitably buffered; titration of the lead, zinc, and copper in this aliquot required 34.78 mL of 0.002700 M EDTA. The copper in a 25.00-mL aliquot was masked with thiosulfate; the lead and zinc were then titrated with 25.62 mL of the EDTA solution.

Cyanide ion was used to mask the copper and zinc in a 100-mL aliquot; 10.00 mL of the EDTA solution was needed to titrate the lead ion. Determine the composition of the brass sample; evaluate the percentage of tin by difference.

Answer 1

Tools we’ll use to find out the composition of brass are as follows.

• Volume and molar concentration of the EDTA: These will give us moles of EDTA used in the titration.

• Coefficients of the complexation reaction equation: These permit us to find moles of in the sample from the moles of EDTA used.

• Equation to calculate number of millimoles:

• Equation to calculate the molar concentration:

Determine the number of moles of lead, zinc, and copper in the sample.

Determine the number of moles of lead and zinc in the sample.

Determine the number of moles of lead in the sample.

Determine the number of moles of zinc in the sample.

Determine the number of moles of copper in the sample.

Calculate the mass percentage of lead in the sample using atomic weight of lead .

Calculate the mass percentage of lead in the sample using atomic weight of zinc .

Calculate the mass percentage of lead in the sample using atomic weight of copper .

Calculate the mass percentage of tin by difference method.

Thus, the composition of the brass sample is , , and .