A solution of a metal ion when treated with `KI` gives a red precipitate which dissolves in excess `KI` to give a colourless solution. Moreover, the solution of metal ion on treatment with a solution of cobalt (II) thiocyanate gives rise to a deep blue crystalline precipitate. The metal ion is

Options

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

Answer

To determine the metal ion based on the given reactions, we can follow these steps:

### Step 1: Analyze the reaction with KI
When a solution of the metal ion is treated with potassium iodide (KI), it produces a red precipitate. The formation of a red precipitate indicates that the metal ion likely forms a compound with iodide ions.

**Hint:** Look for metal ions that form colored precipitates with iodide.

### Step 2: Identify the red precipitate
The red precipitate formed is likely to be mercuric iodide (HgI2), which is known to be scarlet red in color. The reaction can be represented as:
\[ \text{Hg}^{2+} + 2\text{I}^- \rightarrow \text{HgI}_2 \downarrow \]

**Hint:** Consider common metal ions that form red or colored precipitates with iodide.

### Step 3: Examine the behavior in excess KI
The red precipitate (HgI2) dissolves in excess KI to form a colorless solution. This indicates the formation of a soluble complex, which is potassium tetraiodomercurate(II) (K2HgI4). The reaction can be represented as:
\[ \text{HgI}_2 + 2\text{KI} \rightarrow \text{K}_2\text{HgI}_4 \]

**Hint:** Understand that certain metal complexes can dissolve in excess reagents, leading to color changes.

### Step 4: Analyze the reaction with cobalt(II) thiocyanate
When the solution of the metal ion is treated with cobalt(II) thiocyanate (Co(SCN)2), it produces a deep blue crystalline precipitate. This suggests the formation of a complex between mercury and cobalt thiocyanate, likely represented as:
\[ \text{Hg}^{2+} + \text{Co(SCN)}_2 \rightarrow \text{CoHg(SCN)}_4 \downarrow \]

**Hint:** Look for characteristic colors of complexes formed with thiocyanate.

### Step 5: Conclude the identity of the metal ion
Based on the reactions observed:
1. The formation of a red precipitate with KI indicates the presence of Hg²⁺.
2. The dissolution in excess KI to form a colorless solution supports this identification.
3. The formation of a deep blue crystalline precipitate with cobalt(II) thiocyanate further confirms the presence of Hg²⁺.

Thus, the metal ion is **Hg²⁺ (mercury(II) ion)**.

**Final Answer:** The metal ion is **Hg²⁺ (mercury(II) ion)**.