Which of the following cations will form an insoluble red-brown compound with `[Fe(CN)_(6)]^(4-)` ?

Options

A. `Hg^(2+)`
B. `Pb^(2+)`
C. `Cu^(2+)`
D. `Cd^(2+)`

Answer

To determine which cation forms an insoluble red-brown compound with \([Fe(CN)_6]^{4-}\), we can analyze the provided options step by step.

### Step 1: Understand the Reactants
The reactants in this case are the cations and the complex ion \([Fe(CN)_6]^{4-}\). The complex ion is known to react with certain metal cations to form insoluble compounds.

### Step 2: Identify the Cation
From the video transcript, it is mentioned that the cation \(Cu^{2+}\) (copper ion) reacts with \([Fe(CN)_6]^{4-}\) to form a compound. We need to confirm if \(Cu^{2+}\) is the only cation that forms an insoluble compound with this complex ion.

### Step 3: Write the Reaction
The reaction can be written as follows:
\[ 2Cu^{2+} (aq) + [Fe(CN)_6]^{4-} (aq) \rightarrow Cu_2[Fe(CN)_6] (s) \]
This indicates that two copper ions react with one hexacyanoferrate ion to form a solid compound.

### Step 4: Determine the Nature of the Product
The product \(Cu_2[Fe(CN)_6]\) is described as a very insoluble reddish-brown hexa ferrite complex compound. This confirms that \(Cu^{2+}\) forms an insoluble compound with \([Fe(CN)_6]^{4-}\).

### Step 5: Conclusion
Based on the information provided, the cation that forms an insoluble red-brown compound with \([Fe(CN)_6]^{4-}\) is \(Cu^{2+}\).

### Final Answer
The cation that will form an insoluble red-brown compound with \([Fe(CN)_6]^{4-}\) is \(Cu^{2+}\).

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