Reference no: EM13125497

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