The yellow color of an aqueous solution of $\mathrm{K}_{2} \mathrm{CrO}_{4}$ changes to red-orange upon the addition of a few drops of HCl. The red-orange complex, the oxidation state of its central element(s), and the origin of its color, respectively, are:
(a)perchlorate ion, +7, charge transfer
(b)chromium chloride, +3, d-d transition
(c)dichromate ion, +6 and +6, charge transfer
(d)chromic acid, +6, charge transfer
Answer
Answer: C ✓ checked by 4AB · confidence high
The source book printed no answer; this one was worked out and checked — see the explanation.
Explanation
2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O: the red-orange dichromate ion, both chromiums Cr(VI), coloured by O→Cr ligand-to-metal charge transfer (d⁰, no d–d bands).