A 0.3284 g sample of brass (a mix of lead, zinc, copper, and tin) was dissolved in nitric acid. The insoluble SnO2•4H2O (s) was removed by filtration. The combined filtrate and washings were diluted to 500.0 mL. This solution was used as a stock solution.
A 10.00 mL aliquot of the stock solution was suitably buffered and titrated for lead, zinc, and copper using EDTA. A volume of 37.56 mL of 2.500 mM EDTA was required to reach the endpoint.
Separately, a 25.00 mL aliquot of the stock was treated with thiosulfate to mask copper, then the lead and zinc were titrated with EDTA. A volume of 27.67 mL of 2.500 mM EDTA was required to reach the endpoint.
Finally, a 100.0 mL aliquot of the stock was treated with cyanide to mask copper and zinc, then the lead was titrated with EDTA. A volume of 10.80 mL of 2.500 mM EDTA was required to reach the endpoint.
Determine the percentage (by mass) of tin in the sample.
This is my work: https://imgur.com/a/P023M5A
Unsure why i am wrong. any direction in the right way is greatly appreciated.