Sodium nitrite, NaNO 2, is frequently added to processed...

Sodium nitrite, NaNO $_{2},$ is frequently added to processed meats as a preservative. The amount of nitrite ion in a sample can be determined by acidifying to form nitrous acid $\left(\mathrm{HNO}_{2}\right),$ letting the nitrous acid react with an excess of iodide ion, and then titrating the $\mathrm{I}_{3}^{-}$ ion that results with thiosulfate solution in the presence of a starch indicator. The unbalanced equations are $$\text { (1) } \mathrm{HNO}_{2}+\mathrm{I}^{-} \longrightarrow \mathrm{NO}+\mathrm{I}_{3}^{-} \text { (in acidic solution) }$$ $$\text { (2) } \mathrm{I}_{3}^{-}+\mathrm{S}_{2} \mathrm{O}_{3}^{2-} \longrightarrow \mathrm{I}^{-}+\mathrm{S}_{4} \mathrm{O}_{6}^{2-}$$ (a) Balance the two redox equations. (b) When a nitrite-containing sample with a mass of 2.935 g was analyzed, 18.77 $\mathrm{mL}$ of 0.1500 $\mathrm{M} \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3}$ solution was needed for the reaction. What is the mass percent of $\mathrm{NO}_{2}^{-}$ ion in the sample?

Sodium nitrite, NaNO $_{2},$ is frequently added to processed meats as a preservative. The amount of nitrite ion in a sample can be determined by acidifying to form nitrous acid $\left(\mathrm{HNO}_{2}\right),$ letting the nitrous acid react with an excess of iodide ion, and then titrating the $\mathrm{I}_{3}^{-}$ ion that results with thiosulfate solution in the presence of a starch indicator. The unbalanced equations are
$$\text { (1) } \mathrm{HNO}_{2}+\mathrm{I}^{-} \longrightarrow \mathrm{NO}+\mathrm{I}_{3}^{-} \text { (in acidic solution) }$$
$$\text { (2) } \mathrm{I}_{3}^{-}+\mathrm{S}_{2} \mathrm{O}_{3}^{2-} \longrightarrow \mathrm{I}^{-}+\mathrm{S}_{4} \mathrm{O}_{6}^{2-}$$
(a) Balance the two redox equations.
(b) When a nitrite-containing sample with a mass of 2.935 g was analyzed, 18.77 $\mathrm{mL}$ of 0.1500 $\mathrm{M} \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3}$ solution was needed for the reaction. What is the mass percent of $\mathrm{NO}_{2}^{-}$ ion in the sample?

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Key Concepts

Redox Reaction Balancing

This concept involves ensuring that both mass and charge are conserved in a chemical reaction involving electron transfer. In redox reactions, the oxidation and reduction half-reactions are written separately and then balanced for atoms and electrons before being combined to yield the balanced overall reaction.

Half-Reaction Method

A systematic technique used to balance redox reactions by dividing them into two parts: one for oxidation and one for reduction. Each half-reaction is balanced independently in terms of mass and charge, allowing for the correct stoichiometric coefficients to be determined when combining the two.

Iodometric Titration

An analysis method that involves the generation of iodine through redox reactions, which is subsequently titrated with a thiosulfate solution. The precise measurement of the iodine produced, often using a starch indicator to visually detect the endpoint, enables the quantification of the analyte in the sample.

Stoichiometry in Titration

This concept refers to the quantitative relationships derived from a balanced chemical equation. In titrations, the known concentration and volume of a titrant are used to calculate the moles of reactants involved, which in turn is used to determine the concentration or mass percentage of the analyte in a sample.

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