The dark blue colour of the solution formed when excess of ammonia is added to a solution of copper (II) sulphate is due to the presence of the ion:

• A. [Cu(OH)₄(H₂O)₂]²⁺
• B. [Cu(H₂O)₆]²⁺
• C. [Cu(NH₄)₂(H₂O)₄]⁴⁺
• D. [Cu(NH₃)₄(H₂O)₂]²⁺
• E. [Cu(NH₃)₂(H₂O)₄]²⁺

The dark blue color observed when excess ammonia is added to a copper (II) sulfate solution is primarily due to the formation of the complex ion [Cu(NH₃)₄(H₂O)₂]²⁺. This complex results from the interaction between copper ions and ammonia, leading to a distinct blue hue. Here’s a brief overview of the options:

Complex Ion Formation

• A. [Cu(OH)₄(H₂O)₂]²⁺ - This ion is not responsible for the blue color.
• B. [Cu(H₂O)₆]²⁺ - This is the initial hydrated copper ion, which is blue but less intense.
• C. [Cu(NH₄)₂(H₂O)₄]⁴⁺ - This complex does not form under these conditions.
• D. [Cu(NH₃)₄(H₂O)₂]²⁺ - This is the correct answer, leading to a deep blue solution.
• E. [Cu(NH₃)₂(H₂O)₄]²⁺ - This complex is less stable and does not contribute to the intense color.

In summary, the intense dark blue color is attributed to the formation of the [Cu(NH₃)₄(H₂O)₂]²⁺ complex ion when ammonia is added to the copper (II) sulfate solution.