The objective of this lab is to conduct a titration of a sample of bleach with thiosulfate and to determine its end point using a starch indicator. The oxidizing capacity of the bleach will also be derived by determining the percentage by mass of the bleach that is sodium hypochlorite. The overall chemical reaction equation will be created by balancing the two redox reactions. Most bleaches contain the oxidizing agent- the hypochlorite ion. This ion typically comes in the form of a calcium salt [Ca(ClO)2] or sodium salt [NaOCl]. The two oxidation reduction reactions in this experiment are used to determine the oxidizing capacity of bleach. An excess of iodide ions are oxidized to iodine by the hypochlorite ion, and the resulting solution is titrated with sodium thiosulfate and reduced back to iodide. The overall stoichiometry of this reaction requires two moles of the thiosulfate ion to react with every mole of hypochlorite. To prepare for titration, approximately 25 mL water, 0.50 mL of an unknown bleach sample, 3.0 mL of 3.0 M KI, 2.0 mL of 3.0 M H2SO4, and 5 drops of 3% ammonium heptamolybdate are added to a 250 mL beaker. The titration of the aqueous iodine begins as the titrant of 0.050 M Na2S203 is added to the solution. The color of the solution, which is initially brown, fades to light brown and eventually yellow. When yellow, 0.50 mL of the starch indicator is added to the solution, which causes the color to immediately turn a dark blueish-black. Afterwards, the Na2S2O3 titrant is added drop by drop until the solution ultimately becomes colorless, indicating that the end point has been reached. Before the starch is added, the form of the iodine that predominates is the iodide anion (I-). This is because the iodine reacts almost completely with the thiosulfate during the titration, which changes the iodine back to iodide. The form of iodine that predominates after the starch has been added and the endpoint has been reached is iodine (I2). If the starch is added too early on during the titration, the end point becomes harder to detect. The excess iodine would have been absorbed, and it would have taken longer to reach the end point because it would not have been titrated as easily. The main precaution with the addition of sulfuric acid was that adding too much of it would cause the formation of solid iodine crystals in the beaker. The sulfuric acid and catalyst are added to make the reaction go faster. The catalyst must be added immediately after the acid to accelerate the reaction between the iodide and oxidizing agent. If too much is added, the resulting formation of the solid iodine crystals could lead to clumps that are quite hard to dissolve. These clumps will not dissolve and could potentially lead to the discontinuation of the titration. Oxidizing capacity is the strength and ability of an oxidizing agent. In bleach, the effectiveness of whitening is shown through the strength of the oxidizing agent (OCl-). The hypochlorite ion is converted to hypochlorous acid in the presence of a strong acid. This is because the opposite charges from hypochlorite ion and hydrogen ion create the weak acid. The oxidizing capacity of the unknown bleach sample was determined by multiplying the volume of sodium thiosulfate by its molarity. Then, the mole to mole conversion ratio was used to determine the moles of sodium hypochlorite, and this value was then converted to grams of sodium hypochlorite through multiplication of the molecular weight. The resulting value was then divided by the mass of bleach. The oxidizing capacity of my unknown bleach sample was 3.25 % ±0.374. Sources of error that may have affected my results in this experiment include adding the starch a little too soon, not adding the 3% ammonium heptamolybdate immediately after the acid was added, having incorrect weight or volume measurements, and adding extra drops of titrant even after the solution turned yellow.