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Determining Copper in Brass

1st I titrated Na2S2O3 and determined the molarity to be 0.164. I am having trouble with the calculations for the dissolution of brass.

Method Weighed out 0.3026 g of an unknown barass, placed in a 250 mL Erlenmeyer flasks. In fume hood, add 5 mL of 6M HNO_3. Warm the solution on a hot plate until dissolution is complete. When dissolution is complete, add 10 mL of concentrated H_2 SO_4. Continue heating until SO_3fumes disappears. Cool flask for 2 minutes, place in an ice bath. Add 20 mL distilled water. Boil for 2 minutes, ice bath again, and dropwise add NH_3 (aq)until light blue turns dark blue.Add 3M H_2 SO_4 until dark color just disappears. Add 2mL of H_3 PO_4 to each sample. Cover with parafilm and set aside until ready for titration.
Then titrated the dissolved brass - Add 3.9853g KI and titrate immediately with Standard Thiosulfate solution. The color will first be brown or dark yellow-brown. Continue adding the standard thiosulfate solution until a light mustard color appears. Add 5 mL of starch indicator and titrate until color is a lavender-blue. Add 2.0099g of KSCN, mix well. The solution will darken. Continue titration until a white-cream color appears. This is the endpoint - it took 25.48 mL for the white color to emerge.

determine the Cu (%) in brass - I am confused what to calculate for the dissolved brass -
I got this so far -
.3026 brass (1 mol Cu/63.55 g Cu)(1 mol Cu2+/1 mol Cu)(1 mol I2/2 mol Cu2+)(2 mol S2O3/1 mol I2)/.02548 L.

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Solution Preview

Please find attached the detailed explanation. The key is to obtain an equation that relates the moles of Cu to that of your standard solution whose conc. you had as 0.164 M

1. 2Cu2+ + 5I- →2 CuI + I3-
2. I3- + ...

Solution Summary

The solution determines the calculations for the dissolution of brass.