Q57 / 180 · AIIMS 2007 · Chemistry

Wavelength of red light is absorbed by the complex

  • A
    \(\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{2-}\)
  • Correct answer
    B
    \(\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}\)
  • C
    \(\mathrm{CuSO}_4\)
  • D
    \(\mathrm{Cu}(\mathrm{CN})_2\).

Solution

When \(\mathrm{NH}_4 \mathrm{OH}\) is added to the solution containing \(\mathrm{Cu}^{2+}\) ions, deep blue solution is obtained. \(\mathrm{Cu}^{2+}+4 \mathrm{NH}_4 \mathrm{OH} \rightarrow\underset{\text{deep blue solution}}{\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}}+4 \mathrm{H}_2 \mathrm{O}\) Hydrated cupric compounds absorb radiations corresponding to red light and the transmitted colour is greenish blue (which is complementary to red colour). Thus, cupric compounds give greenish-blue colour.

Hence, the correct answer is option: 'B'.

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