Question

A pale green crystalline metal salt of (X) dissolves freely in water. It gives a brown precipitate on the addition of aq. \[NaOH\]. The metal salt solution also gives a black precipitate on bubbling \[{{H}_{2}}S\] in aqueous medium. An aqueous solution of the metal salt decolourises the pink colour of the permanganate solution. The metal in the metal salt solution is:
A. \[Fe\]
B. \[Pb\]
C. \[Cu\]
D. \[Al\]

Answer 1

Hint: To solve this question we have to keep certain things in mind for example, if a salt reacts to give a product with aq. \[NaOH\] then it should be acidic in nature because then only it will be able to react with a base. Copper salts are generally blue in colour, lead salts are white or black, iron salts are greenish and aluminium salts are white.

Complete answer:
The metal in the metal salt solution is iron \[\left( Fe \right)\].
The pale green crystalline metal salt is ferrous sulphate \[\left( FeS{{O}_{4}} \right)\]. It readily dissolves in water and on standing in water it gives a brown ppt. of basic ferric sulphate \[Fe(OH)(S{{O}_{4}})\].

\[4FeS{{O}_{4}}+2{{H}_{2}}O+{{O}_{2}}\to 4Fe(OH)(S{{O}_{4}})\]

The metal salt solution also gives a black ppt. on bubbling \[{{H}_{2}}S\].
\[F{{e}^{2+}}+{{H}_{2}}S\to FeS+2{{H}^{+}}\]
An aqueous solution of the metal salt decolourised the pink colour of the permanganate solution by forming colourless products.

\[10FeS{{O}_{4}}+8{{H}_{2}}S{{O}_{4}}+2KMn{{O}_{4}}\to 5F{{e}_{2}}{{(S{{O}_{4}})}_{3}}+2MnS{{O}_{4}}+{{K}_{2}}S{{O}_{4}}+8{{H}_{2}}O\]

Hence, the correct answer is that the metal (X) present in the solution which satisfies all the conditions is iron \[\left( Fe \right)\] which is option A.

Note: The decolourisation of the permanganate solution is a redox reaction in which the permanganate ion gets reduced and it oxidises the other metal ion present into its higher stable oxidation state.