Question

An aqueous solution of a salt X turns blood red on treatment with $SC{{N}^{-}}$ and blue on treatment with ${{K}_{4}}[Fe(C{{N}_{6}})]$. X also gives a positive chromyl chloride test. The salt X is:
A)$Cu{{(N{{O}_{3}})}_{2}}$
B)$FeC{{l}_{3}}$
C)$CuC{{l}_{2}}$
D)$Fe{{(N{{O}_{3}})}_{3}}$

Answer 1

Hint: The answer here is based upon the fact that the chromyl chloride test where it is the confirmation test used for the identification of the chloride ions that is $C{{l}^{-}}$ ions present. The answer lies here.

Complete step by step answer:
In the lower classes of chemistry, we have come across various tests used for the confirmation of the functional group present and also for the metal ions present which we have carried out in the laboratories.
Let us now discuss in brief about the chromyl chloride test and its use.
- Chromyl chloride test is used for the qualitative analysis for the confirmation of the presence of chloride ions. Here, the salt is treated with potassium dichromate and sulphuric acid where orange yellow fumes of chromyl chloride evolve which confirms the presence of the chloride group in the given salt.
In the above question, the salt gives positive chromyl chloride test which means that the salt has chloride ions in it. Thus options A) and D) are ruled out.
Now, the thiocyanate ion reacts with iron in +3 oxidation state and thus forms blood red ferric thiocyanate and the reaction is,
\[FeC{{l}_{3}}+3SC{{N}^{-}}\to [Fe{{(CNS)}_{3}}]+3C{{l}^{-}}\]
also ferric chloride reacts with potassium ferrocyanide as,
\[4FeC{{l}_{3}}+3{{K}_{4}}[Fe{{(CN)}_{6}}]\to F{{e}_{4}}{{[Fe{{(CN)}_{6}}]}_{3}}+12KCl\]
This precipitate ferric ferrocyanide is blue in colour.
Also this compound gives a positive test for chromyl chloride test as it has chloride ions.
On the other hand, $CuC{{l}_{2}}$ with thiocyanate ion form black precipitate of copper (II) thiocyanate.

Thus, the correct answer is option B) $FeC{{l}_{3}}$.

Note: Note that thiocyanate ion is the conjugate base of thiocyanate and is colourless and this is used widely as important spectro photometric reagent and this reagent is principally used for the determination of Fe (III), Mo, W, Ti and several other ions.