Chapter 21, Problem 21.189QP

The amount of sodium hypochlorite in a bleach solution can be determined by using a given volume of bleach to oxidize excess iodide ion to iodine, because the reaction goes to completion. The amount of iodine produced is then determined by titration with sodium thiosulfate, Na₂S₂O₃, which is oxidized to sodium tetrathionate, Na₂S₄O₆. Potassium iodide was added in excess to 5.00 mL of bleach (density = 1.00 g/mL). This solution, containing the iodine released in the reaction, was titrated with 0.100 M Na₂S₂O₃. If 34.6 mL of sodium thiosulfate was required to reach the endpoint (detected by disappearance of the blue color of the starch iodine complex), what was the mass percentage of NaClO in the bleach?