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Question: The dark blue colour of the solution formed when excess of ammonia is added to a solution of copper ...

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+{{[Cu{{(OH)}_{4}}{{({{H}_{2}}O)}_{2}}]}^{2+}}
B. [Cu(H2O)6]2+{{[Cu{{({{H}_{2}}O)}_{6}}]}^{2+}}
C. [Cu(NH4)2(H2O)4]4+{{[Cu{{(N{{H}_{4}})}_{2}}{{({{H}_{2}}O)}_{4}}]}^{4+}}
D. [Cu(NH3)4(H2O)2]2+{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}O)}_{2}}]}^{2+}}
E. [Cu(NH3)2(H2O)4]2+{{[Cu{{(N{{H}_{3}})}_{2}}{{({{H}_{2}}O)}_{4}}]}^{2+}}

Explanation

Solution

When an excess amount of ammonia is going to react with copper (II) sulphate then it forms a coordination complex of copper. The complex of copper is the reason for the formation of the blue color.

Complete step-by-step answer: - In the question it is given that an excess amount of ammonia is added to copper (II) sulphate.
- The chemical formula of ammonia is NH3N{{H}_{3}} and the chemical formula of copper (II) sulphate is CuSO4CuS{{O}_{4}}.
- The chemical reaction of copper (II) sulphate with ammonia is as follows.
[Cu(H2O)6]2+Copper sulphate solution +4NH3→[Cu(NH3)4(H2O2)2]2+Blue color complex +4H2O\underset{\text{Copper sulphate solution}}{\mathop{{{[Cu{{({{H}_{2}}O)}_{6}}]}^{2+}}}}\,+4N{{H}_{3}}\to \underset{\text{Blue color complex}}{\mathop{{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}{{O}_{2}})}_{2}}]}^{2+}}}}\,+4{{H}_{2}}O
- In the above reaction one mole of copper sulphate in water reacts with four moles of ammonia and forms a blue color complex with the molecular formula of [Cu(NH3)4(H2O2)2]2+.{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}{{O}_{2}})}_{2}}]}^{2+}}.
- The formed complex [Cu(NH3)4(H2O2)2]2+{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}{{O}_{2}})}_{2}}]}^{2+}} is blue color.
- Therefore the molecular formula of the complex formed when excess ammonia reacted with copper sulphate solution is [Cu(NH3)4(H2O2)2]2+{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}{{O}_{2}})}_{2}}]}^{2+}}.

So, the correct option is D.

Note: Copper has a capability to form a complex with different ligands and the formed complexes are called coordination compounds. The color of the coordination compounds is going to depend on the number of unpaired electrons present on the central metal in the coordination complex. The color of the complex is [Cu(NH3)4(H2O2)2]2+{{[Cu{{(N{{H}_{3}})}_{4}}{{({{H}_{2}}{{O}_{2}})}_{2}}]}^{2+}} is due to the presence of unpaired electrons in copper metal atom.