When an aqueous solution of copper (II) sulphate is saturated with ammonia, the blue compound crystallises on evaporation. The formula of this blue compound is:

Options

A. `[Cu(NH_3)SO_(4).H_(2)O` (square planar)
B. `[Cu(NH_(3))_(4)]SO_(4)`(Tetrahedral)
C. `[Cu(NH_2)_6 ] SO_(4)`(Octahedral)
D. `[Cu(SO_(4))(NH_3)_5]` (Octahedral)

Answer

To determine the formula of the blue compound that crystallizes when an aqueous solution of copper (II) sulfate is saturated with ammonia, we can follow these steps:

### Step 1: Understand the Reaction
When copper (II) sulfate (CuSO4) is mixed with ammonia (NH3), a complex ion is formed. The ammonia acts as a ligand and coordinates with the copper ion.

**Hint:** Remember that ligands are molecules or ions that can donate a pair of electrons to a central metal atom to form a coordination complex.

### Step 2: Identify the Copper Ion
In the solution, copper exists in the +2 oxidation state (Cu²⁺). This is important because it determines how many ligands can coordinate with the copper ion.

**Hint:** The oxidation state of the metal ion is crucial for understanding its coordination chemistry.

### Step 3: Determine the Coordination Number
Copper (II) typically forms complexes with a coordination number of 4 or 6. In this case, with ammonia as a ligand, it forms a square planar complex, which is common for d⁹ metal ions like Cu²⁺.

**Hint:** The geometry of the complex can help you predict the coordination number.

### Step 4: Write the Formula of the Complex
The ammonia ligands coordinate with the copper ion. Since there are four ammonia molecules coordinating with one copper ion, the formula for the complex ion will include Cu and NH3. Additionally, the sulfate ion (SO4²⁻) is present in the compound.

**Hint:** Pay attention to the number of ligands and the counter ions in the final formula.

### Step 5: Include Water of Crystallization
When the complex crystallizes, it often includes water molecules as part of the crystal structure. In this case, we have two water molecules (2H2O) associated with the complex.

**Hint:** Water of crystallization is common in hydrated salts and should be included in the final formula.

### Step 6: Combine All Components
Putting it all together, the formula for the blue compound formed is Cu(NH3)4SO4·2H2O.

**Hint:** Make sure to check the final formula for correct stoichiometry and the presence of water of crystallization.

### Final Answer
The formula of the blue compound is **Cu(NH3)4SO4·2H2O**.