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When excess ammonia is added to a copper sulphate solution, a deep blue colour appears because of the formation of which species?

1. Cu2+
2. Cu(NH3)4²+
3. Cu(NH3)6²+
4. Cu(NH3)2²+

Correct answer: Cu(NH3)4²+

Solution

Excess NH3 with CuSO4 forms the deep blue tetraamminecopper(II) ion [Cu(NH3)4]^2+. Cu(II) in aqueous ammonia takes 4 NH3 ligands in the square-planar/equatorial plane giving the characteristic intense blue colour.

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