 We're going to do a titration to determine the mass percent of sodium hypochloride in commercial bleach. We're going to prepare a diluted solution of bleach and pipette a 25-milliliter sample of that dilute solution. We're going to react that solution with an excess of potassium iodide. Iodine will be produced, we'll titrate the iodine produced with a standardized solution of sodium thiosulfate. The volume and the molarity of the sodium thiosulfate solution used in the titration will allow you to calculate the moles of thiosulfate used, the moles of iodine produced, and the moles of hypochloride in your dilute bleach solution. This will allow you to calculate the number of grams of sodium hypochloride in the dilute solution and the number of grams of sodium hypochloride in the concentrated solution. You can use that data to calculate the mass percent of sodium hypochloride in the bleach. We're going to pipette 5 milliliters of the commercial bleach solution into a 100-milliliter volumetric flask. We're going to add distilled water and we're going to fill the flask until the bottom of the meniscus reaches the line on the volumetric flask. We will then stop for the flask and mix the solution so that it is homogeneous. We're going to pipette 25 milliliters of our diluted bleach solution into a 250-milliliter early Meyer flask. We're going to add a small amount of water, roughly about 20 milliliters of water to the flask to give us a little bit of additional volume. We're going to add 2 milliliters of 3-mower HCl to our reaction mixture. We're there going to add 2 grams of solid potassium iodide to the reaction mixture. The solution will turn a reddish-orange color due to the formation of iodine. The hypochloride in the bleach oxidizes I- to I2. We're going to titrate the iodine produced with a standardized solution of sodium thiosulfate. The sodium thiosulfate will reduce the iodine back to iodide. We'll take an initial burette reading. We'll slowly add the thiosulfate solution until the reaction mixture is a real pale yellow color. We should rinse down the walls of the flask periodically to make sure that everything has a chance to react. We're going to titrate to a very light yellow color and then we're going to add several drops of starch indicator solution. The solution will turn blue due to the formation of a complex between the iodine and the starch. We will continue the titration until the blue color disappears. We're going to take a final burette reading and determine the total number of milliliters of sodium thiosulfate solution added to the reaction mixture.