A commercial liquid bleach contains aqueous sodium hypochlorite of unknown concentration

CHEMISTRY · Multi-part · CBSE · Chemistry · Some Basic Concepts of Chemistry · Some Basic Concepts of Chemistry

Problem

A commercial liquid bleach contains aqueous sodium hypochlorite

(\text{NaOCl})

of unknown concentration. A 10.00 mL sample of the bleach is transferred to a 250.0 mL volumetric flask and diluted to the mark with deionized water. A 25.00 mL aliquot of the diluted solution is placed in an Erlenmeyer flask containing excess potassium iodide and dilute acid, producing iodine by the net reaction

\text{OCl}^- + 2\,\text{I}^- + 2\,\text{H}^+ \rightarrow \text{I}_2 + \text{Cl}^- + \text{H}_2\text{O}

The liberated

\text{I}_2

is titrated with 0.150 M sodium thiosulfate according to

\text{I}_2 + 2\,\text{S}_2\text{O}_3^{2-} \rightarrow 2\,\text{I}^- + \text{S}_4\text{O}_6^{2-}

The titration requires 32.65 mL of the thiosulfate solution to reach the starch end point. (a) Calculate the number of moles of

\text{S}_2\text{O}_3^{2-}

delivered from the buret. (b) Determine the number of moles of

\text{I}_2

that reacted with the thiosulfate. (c) From the stoichiometry of the iodine–hypochlorite reaction, compute the number of moles of

\text{OCl}^-

present in the 25.00 mL aliquot. (d) Calculate the molarity of

\text{OCl}^-

in the original commercial bleach prior to any dilution. (e) Propose one significant source of systematic error in this procedure that could lead to an overestimation of the bleach concentration and explain the chemical basis for its effect.

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