Answer to Question #154949 in General Chemistry for Gusty

Question

1. What are the oxidizing and reducing agents in this experiment? Write chemical equations.

2. Why starch is not added earlier in the titration at the same time as KI.

3. 20.0 mL sample of bleach is diluted to 100 mL. A 25.0 mL sample of this solution is analyzed according to the procedure of this experiment. If 40.0 mL of 0.15 M Na2S2O3 is needed to reach the end point, what is the percent by weight of NaOCl in the original sample? Assume the density of bleach to be 1.08 g/mL.

4. A bleach solution has a weight percent of 6.5% (density = 1.09 g/L) was diluted to 0.040 M, calculate the dilute factor?

Answer 1

1)The permanganate ion removes electrons from oxalic acid molecules and thereby oxidizes the oxalic acid. Thus, the MnO₄^- ion acts as an oxidizing agent in this reaction. Oxalic acid, on the other hand, is a reducing agent in this reaction. By giving up electrons, it reduces the MnO₄^- ion to Mn²⁺.

Oxidation = H₂C₂O₄ $\to$ CO₂

Reduction = MnO$_4^-$ $\to$ Mn⁺²

2 MnO₄^-(aq) + 5 H₂C₂O₄(aq) + 6 H⁺(aq)$\to$  10 CO₂(g) + 2 Mn²⁺(aq) + 8 H₂O(l)

2)The starch indicator is added to the solution near the end of the titration, at the point where dilute iodine imparts a pale yellow color to the solution. There are two reasons why the indicator is not added at the beginning of the titration when the iodine concentration is high. First, a diffuse endpoint would result from the slow dissociation of the starch-iodine complex if a large amount of iodine were absorbed in the starch. Second, iodometric titrations are carried out in strongly acid media, a situation that promotes the reaction between oxidizing agents and iodide. Unfortunately starch has a tendency to hydrolyze (decompose) in acidic media, destroying its indicator qualities.

3) 6.20833333%

4) 23.7751678