1. a Write the balanced equations for the following oxidation–reduction reactions that you will encounter in this experiment: a. The reduction of ClO− by I− in acidic solution ?b. The reduction of I2 by S2O32−in acidic solution 2.a. What conversion factor will enable you to calculate the number of moles of ClO– ions from the number of moles of S2O32− ions used in the titration? b. A 10.0-mL sample of aqueous NaOCl is treated with excess KI in an acidic solution. The quantity of iodine that is liberated is such that 28.02 mL of 0.0250 M Na2S2O3 solution must be added to cause the disappearance of the dark blue color due to the starch indicator. What is the molarity of the solution of NaOCl? 388Cleaning and Filling your Buret1. Instructions for using a buret can be found in the Introduction section of this manual. Cleanyour buret according to the directions, and fill it with some of the Na₂S₂O3 solution.3.4.Doing the Trial Titration1. Pipet 10.0 mL of the diluted bleach solution into a clean 125-mL Erlenmeyer flask.2.Add one of the samples of solid KI to the flask, followed by 20 mL of distilled water from agraduated cylinder and 20 drops of 2 M HCl. Swirl to obtain a homogeneous solution.5.6.7.8.CAUTION: Undiluted bleach and hydrochloric acid can causechemical burns and ruin your clothes. In addition, bleach can beespecially irritating to the eyes. If you spill either of thesesolutions on you, wash the contaminated area thoroughly andimmediately report the incident to your laboratory instructor.You may require further treatment. Be sure to wear approvedchemical splash goggles.3.4.5.Experiment 21B6.7.8.9.Record the initial buret reading to the closest 0.01 mL.Place the flask under the buret with the capillary tip inside the mouth of the flask. Insert a piece ofwhite paper under the flask.Doing the Exact Titrations1. Repeat Steps 1 through 4 of the procedures for the trial titration.2.Subtract 2 mL. from the volume found in the trial titration. Rapidly add this new volume to theflask from the buret.Add 40 drops of the 0.2% starch indicator to the flask.Rinse the walls of the flask with distilled water from a plastic wash bottle.Continue the titration on a drop-by-drop basis. Swirl the flask rapidly after each addition.Remember that the endpoint will occur when one drop results in a colorless solution.While swirling, add increments of about 1 mL of the Na₂S₂O3 solution to the flask. Continue untilthe brown color fades to yellow.Add 40 drops of a 0.2% starch solution to the flask.Continue to add 1-mL portions of the Na₂S₂O3 solution until one addition causes the solution tobecome colorless.Record the final buret reading to the nearest 0.01 mL, and calculate the approximate volume ofthe Na₂S₂O3 solution required for the titration.Record the final buret reading to the nearest 0.01 mL.Repeat the procedure with a second sample of the diluted bleach solution.If the volumes at the endpoints differ by more than 0.15 mL. (about 3 drops) or some other amountspecified by your instructor, repeat the titrations with additional samples of the diluted bleachuntil the required precision is obtained.Calculate and record the molarity of the diluted bleach solution. Obtain the mean molarity. 21B. The Strength of a Laundry BleachIntroductionHypochlorous acid (HCIO) is one of the important chlorine oxoacids (Ebbing/Gammon, Section21.9). Solutions of sodium hypochlorite (NaOCI), a salt of that acid, are sold as laundry bleach. Thehypochlorite anion (CIO) is a strong oxidizing agent, but not as strong as ClO₂, CIOs, and ClO4.PurposeIn this experiment, you will use an oxidation-reduction titration to determine the quantity of NaOCIin a commercial bleach.Concept of the ExperimentAs you will see, you can determine the quantity of the CIO ion in a solution through two oxidation-reduction reactions. First, a known quantity of this anion is reduced to CI ions in an acidic solution,using excess potassium iodide. The I ions are oxidized to I₂ in this reaction. The solution that results isbrown because that is the color of I₂ in water. Second, the I2 is reduced to L during a titration withasolution of sodium thiosulfate. Thiosulfate anions (S₂O) are oxidized to tetrathionate ions (S₂0²)in this reaction.You may have found this description somewhat confusing, but it should become clear after youhave balanced the equations for the two oxidation-reduction reactions in the PrelaboratoryAssignment. Moreover, the relationship between the original quantity of ClO ions and the quantityof S₂0 ions used in the titration will be evident after these equations are balanced.Although you could use the disappearance of the color due to aqueous I₂ to detect the endpoint of thetitration, this technique would not be very sensitive. Instead, you will use starch as an indicator.Starch reacts with 12 to form a dark blue color. This reaction is reversible. Consequently, the bluecolor fades during the course of the titration as I is consumed. The endpoint occurs when one drop ofthe Na₂S₂O3 solution causes the color to change from blue to colorless. A trial titration will enable youto locate the endpoints of subsequent titrations more easily.ProcedureGetting Started1.2.Obtain a 10-mL transfer pipet and a 50-mL buret.Obtain about 70 mL of a 0.0250 M solution of Na₂S₂O3 and 3 samples of solid KI. Each ofthese samples should have a volume of about 1 cm³. They can be stored on pieces of waxedpaper.3.A buret containing the bleach solution will be available for general use. This solution must bediluted, however. Record the initial buret reading to the nearest 0.01 mL. Carefully add about 3mL of the solution to a 100-mL volumetric flask. (A 100-mL graduated cylinder may be used ifa volumetric flask is not available.) Record the final buret reading to the nearest 0.01 mL, andcalculate the volume of undiluted bleach used. Add distilled water to the flask until the bottomof the meniscus coincides with the etched line on the flask. Add the last 0.5 mL with a medicinedropper. Insert a stopper in the flask, and mix the solution thoroughly.4.Be careful in your handling of the solutions used in this experiment. Obtain directions from yourlaboratory instructor regarding proper disposal of all solutions used during this experiment.

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10th Edition

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Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste

Chapter18: Electrochemistry

Section: Chapter Questions

Problem 159MP: A galvanic cell is based on the following half-reactions: In this cell, the copper compartment...