Chemistry - Salt Analysis Question with Solution | TestHub

The question asks which anion does NOT reduce yellow ${\mathrm{FeCl}}_3$ (containing ${\mathrm{Fe}}^{3+}$) to green ${\mathrm{FeCl}}_2$ (containing ${\mathrm{Fe}}^{2+}$). This requires identifying which anion is NOT a reducing agent.

A. ${\mathrm{S}}_2{\mathrm{O}}_3{}^{2-}$ (thiosulfate) is a reducing agent, often oxidized to ${\mathrm{S}}_4{\mathrm{O}}_6{}^{2-}$ or ${\mathrm{SO}}_4{}^{2-}$.

B. ${\mathrm{S}}^{2-}$ (sulfide) is a strong reducing agent, oxidized to elemental sulfur or sulfate.

C. ${\mathrm{NO}}_2{}^{-}$ (nitrite) can act as a reducing agent, being oxidized to ${\mathrm{NO}}_3{}^{-}$.

D. ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ (acetate) is generally not a reducing agent in this context. It is a weak base and forms complexes but does not readily reduce ${\mathrm{Fe}}^{3+}$ to ${\mathrm{Fe}}^{2+}$.

Therefore, ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$ does not convert ${\mathrm{FeCl}}_3$ to ${\mathrm{FeCl}}_2$.

The final answer is $\boxed{D}$