Lecture 29 11/11/05

Winkler method to measure O2 2Mn2+(aq) + 4OH-(aq) + O2(aq)  2MnO2 (s) + 2H2O MnO2(s) + 3I-(aq) +4H3O+(aq)  Mn2+(aq) + I3-(aq) + 6H2O I3-(aq) + 2S2O32-  3I-(aq) + S4O62-

Using the Winkler Method (equations below), find the mg/L of O2 (FM=32g/mol), if 10 mL of 0.1 M Na2S2O3 was needed to titrate a 1 L sample. 2Mn2+(aq) + 4OH-(aq) + O2(aq)  2MnO2 (s) + 2H2O MnO2(s) + 3I-(aq) +4H3O+(aq)  Mn2+(aq) + I3-(aq) + 6H2O I3-(aq) + 2S2O32-  3I-(aq) + S4O62-

12-47 A 10.231-g sample of window cleaner containing ammonia was diluted with 39.466 g of water. Then 4.373 g of solution were titrated with 14.22 mL of 0.1063 M HCl to reach a bromocresol green end point. Find the weight percentage of NH3 (FM 17.031) in the cleaner.

16-25 A 3.026-g portion of a copper(II) salt was dissolved in a 250-mL volumetric flask. A 50.0-mL aliquot was analyzed by adding 1 g of KI and titrating the liberated iodine with 23.33 mL of 0.04668 M Na2S2O3. Wt% of copper in salt? Copper = 63.5 g/mol

14-22

2Cu2+ + 5I-  2CuI(s) +I3- e- + Cu2+ + I-  CuI(s) 3I-  I3- + 2e-