The concentration of an aqueous solution of NaOCl (sodium hypochlorite; the active ingredient in household bleach) can be determined by a redox titration with iodide ion in acidic solution:
Assume that the blue spheres in the buret represent I-ions, the red spheres in the flask represent OCl-ions, the con-centration of the I-ions in the buret is 0.120 M, and the volumes in the buret and the flask are identical. What is the concentration of NaOCl in the flask? What percentage of the I-solution in the buret must be added to the flask to react with all the OCl-ions?

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Write the balanced redox reaction between NaOCl and I- in acidic solution: OCl^- + 2 I^- + 2 H^+ -> I2 + Cl^- + H2O.

Determine the number of moles of I- in the buret using the concentration (0.120 M) and the volume (assume V).

Count the number of OCl- ions in the flask from the image (10 red spheres) to determine the moles of OCl-.

Use the stoichiometry of the balanced equation to find the moles of I- needed to react with the moles of OCl-.

Calculate the percentage of the I- solution in the buret that must be added to the flask by comparing the moles of I- needed to the total moles of I- available.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Redox Reactions

Redox reactions involve the transfer of electrons between two species, resulting in oxidation and reduction processes. In this titration, iodide ions (I-) are oxidized to iodine (I2), while hypochlorite ions (OCl-) are reduced. Understanding the electron transfer is crucial for determining the stoichiometry of the reaction and calculating concentrations.

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. In this case, the volume of iodide ions added from the buret to the flask containing sodium hypochlorite is measured to find the endpoint of the reaction. The relationship between the volumes and concentrations of the reactants allows for the calculation of the unknown concentration of NaOCl.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on balanced equations. It allows for the determination of the amount of I- needed to completely react with the OCl- in the flask. By using the molarity and volume of the solutions, one can derive the concentration of NaOCl and the percentage of I- solution required for the reaction.

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