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Question
Which one is colourless anion?
Options
\[\ce{HCO^-_3}\]
\[\ce{Cr2O^2-_4}\]
\[\ce{Cr2O^2-_7}\]
\[\ce{MnO^-_4}\]
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Solution
\[\ce{HCO^-_3}\]
Explanation:
The bicarbonate ion \[\ce{(HCO^-_3)}\] is colourless because it is composed of representative elements (hydrogen, carbon, and oxygen) which do not possess partially filled d-orbitals or undergo charge-transfer transitions in the visible light spectrum. In contrast, the other options contain transition metals that exhibit vibrant colors due to charge-transfer transitions chromate \[\ce{(CrO^{2-}_4)}\] is yellow, dichromate \[\ce{(Cr2O^{2-}_7)}\] is orange, and permanganate \[\ce{(MnO^-_4)}\] is deep purple.
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