Chapter 4, Problem 4.155MP

Sodium nitrite, ${\text{NaNO}}_{\text{2}}$ , 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 $\left({\text{HNO}}_{\text{2}}\right),$ letting the nitrous acid react with an excess of iodide ion, and then titrating the ${\text{I}}_3{}^{-}$ ion that results with thiosulfate solution in the presence of a starch indicator. The unbalanced equations are
(1) ${\text{HNO}}_2+{\text{I}}^{-}\to \text{NO}+{\text{I}}_3{}^{-}\left(\text{in acidic solution}\right)$
(2) ${\text{I}}_3{}^{-}+{\text{S}}_{\text{2}}{\text{O}}_3{}^{2-}\to {\text{I}}^{+}+{\text{S}}_{\text{4}}{\text{O}}_{\text{6}}{}^{2-}$
(a) Balance the two redox equations.
(b) When a nitrite-containing sample with a mass of 2.935 g was analyzed, 18.77 ml of $0.\text{15}00{\text{ M Na}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{3}}$ solution was needed for the reaction. What is the mass percent of ${\text{NO}}_{\text{2}}{}^{-}$ ion in the sample?