Chemistry - Salt Analysis Question with Solution | TestHub

Question Explanation: Identify the anion in solution 'A' using reaction tests.

Concept: Qualitative identification of the thiosulfate ion $S_2{O}_3^{2-}$ through complexation and redox reactions.

Solution:

Test 1: With dilute HCl Thiosulfates react with dilute acids to evolve sulfur dioxide gas and precipitate elemental sulfur, which appears as a white turbidity (colloidal sulfur).

$$S_2{O}_3^{2-}(aq)+2H^{+}(aq)\to S{O}_2(g)+S(s)\left(White\right)+H_{2}O(l)$$

Test 2: With $FeC{l}_3$

Adding ferric chloride to a thiosulfate solution initially produces a dark violet coloration due to the formation of a ferric thiosulfate complex$[Fe(S_2{O}_3{)}_2{]}^{-}$. On standing, this color disappears as $F{e}^{3+}$ is reduced to $F{e}^{2+}$ (which is light green in solution) and thiosulfate is oxidized to tetrathionate ($S_4{O}_6^{2-}$).

Test 3: With KCN and $FeC{l}_3$Thiosulfate reacts with cyanide ions to form thiocyanate ($SC{N}^{-}$). When $FeC{l}_3$ is then added, it forms the characteristic blood-red iron(III) thiocyanate complex

$$[Fe(SCN){]}^{2+}$$

.

$$S_2{O}_3^{2-}(aq)+C{N}^{-}(aq)\to SC{N}^{-}(aq)+S{O}_3^{2-}(aq)$$

$F{e}^{3+}(aq)+SC{N}^{-}(aq)\to [Fe(SCN){]}^{2+}(aq)$ (blood red)

The anion present in aqueous solution 'A' is $S_2{O}_3^{2-}$ (Thiosulfate).

Final Answer: The correct option is (c).