Which of the following molecules is colourless?

Which of the following molecules is colourless?
  1. $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$ (crystal)
  2. $\mathrm{CuSO}_4$ (anhydrous)
  3. $\left[\mathrm{Cu}\left(\mathrm{NH}_3ight)_4ight]^{2+}(\mathrm{aq})$
  4. $\left[\mathrm{CuCl}_4ight]^{2-}(\mathrm{aq})$

Solution

Hydrated copper sulphate $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$ is blue in colour. The ligand (water) molecules causes splitting of $d$-orbitals. This facilitated $d$ - $d$ transition and colour. Anhydrous copper sulphate $\mathrm{CuSO}_4$ is colourless. In the absence of ligand (water) molecules, splitting of $d$-orbitals is not possible. Hence, $d$ - $d$ transition is not possible. Hence, option (b) is correct. ^

Asked in: JEE-TOPICTESTS-CHEMISTRY

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