`CoS` (black) obtained in group `IV` of salt analysis is dissolved in aqua regia and is treated with an excess of `NaHCO_(3)` and then `Br_(2)` water.An apple green coloured stable complex is formed. It is:

Options

A. sodium cobaltocarbonate
B. sodium cobaltibromide
C. sodium cobalticarbonate
D. sodium cobaltobromide

Explanation

To solve the problem step by step, we will analyze the reactions involving cobalt sulfide (CoS), aqua regia, sodium bicarbonate (NaHCO₃), and bromine water (Br₂).

### Step 1: Dissolution of CoS in Aqua Regia
Cobalt sulfide (CoS) is treated with aqua regia, which is a mixture of hydrochloric acid (HCl) and nitric acid (HNO₃). The reaction can be represented as follows:

\[ \text{CoS} + \text{HCl} + \text{HNO}_3 \rightarrow \text{CoCl}_2 + \text{NO} + \text{S} + \text{H}_2\text{O} \]

**Hint:** Aqua regia is a powerful oxidizing agent that can dissolve noble metals and sulfides.

### Step 2: Formation of Cobalt Chloride
From the above reaction, cobalt chloride (CoCl₂) is formed along with nitrogen monoxide (NO) and sulfur (S). The cobalt chloride is a soluble compound that can further react with sodium bicarbonate.

**Hint:** Cobalt chloride is typically blue in solution but can form different complexes.

### Step 3: Reaction with Sodium Bicarbonate
Next, cobalt chloride reacts with an excess of sodium bicarbonate (NaHCO₃). The reaction can be simplified as follows:

\[ \text{CoCl}_2 + 2 \text{NaHCO}_3 \rightarrow \text{Na}_2\text{Co(CO}_3)_2 + 2 \text{NaCl} + \text{H}_2\text{O} \]

This reaction leads to the formation of sodium cobaltic carbonate, which is a complex compound.

**Hint:** Sodium bicarbonate acts as a source of carbonate ions that help in the formation of the complex.

### Step 4: Treatment with Bromine Water
The sodium cobaltic carbonate complex is then treated with bromine water (Br₂). The reaction can be expressed as follows:

\[ \text{Na}_2\text{Co(CO}_3)_2 + \text{Br}_2 \rightarrow \text{Na}_2\text{Co(CO}_3)_2 + \text{Br}_2 \]

This step leads to the stabilization of the complex and may also release carbon dioxide (CO₂) and water (H₂O) as by-products.

**Hint:** Bromine water is used to oxidize the cobalt, which helps stabilize the complex.

### Step 5: Identification of the Complex
The final complex formed is identified as sodium cobaltic carbonate, which has an apple green color.

**Hint:** The characteristic color of the complex is significant for identification in qualitative analysis.

### Conclusion
The apple green colored stable complex formed is sodium cobaltic carbonate, represented as:

\[ \text{Na}_2\text{Co(CO}_3)_2 \]

**Final Answer:** Sodium cobaltic carbonate (Na₂Co(CO₃)₂) is the apple green colored stable complex formed.