Unique # 51070
The main purpose of the experiment involves two oxidation-reduction reactions to calculate the oxidizing capacity of a sample of unknown bleach. In order to determine the volume of Na2S2O3 added, students will conduct a titration of bleach with thiosulfate with addition of a starch indicator to find the end point of the titration. Moreover, the oxidizing capacity of bleach is calculated with the percentage by mass of NaOCl in the unknown bleach sample. The overall chemical reaction throughout the experiment will be balanced with the two oxidation-reduction reactions.
Within the household chlorine bleaches, sodium hypochlorite (NaOCl) and calcium hypochlorite Ca(ClO)2 are the oxidizing agents. Before titration, the mass of the bleach is weighed out for each trial and recorded, approximately 0.4-0.6 grams. After the bleach sample is weighed, bleach is poured into a beaker with several aliquots of distilled water to completely wash the bleach out. Next, 3 M KI is added to the beaker, then 3 M H2SO4 is poured into the bleach with 5 drops of 3% ammonium molybdate catalyst immediately subsequent of acid addition. Titration begins with Na2S2O3 added into the bleach solution until the solution turns completely yellow, then drops of starch indicator is added and mixed. The starch will turn the solution into a dark blue-black color, and then titration should continue until the solution turns colorless at the end point and the buret reading will be recorded. Before the starch is added, triiodide is the form of iodide that is present; after reaching endpoint, iodine reduces and returns to iodide ions.
A precautionary procedure to be considered is to ensure the color of the solution is transformed into a complete yellow color before adding the starch. If the starch is added before it is yellow, the reaction of the dark-blue complex would be difficult to reverse and would require excessive amount of time to dissociate; thus it would be...
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