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Chromite ore is fused with Na2CO3 in excess air to prepare K2Cr2O7. Which of the following statements about this process is correct?

1. Chromite ore contains iron in the +2 oxidation state.
2. The oxidation number of Cr in the fusion product is +6.
3. In the fusion product, all Cr-O bond lengths are equal.
4. In the fusion product, Cr has no unpaired electron but is still coloured.

Correct answer: The oxidation number of Cr in the fusion product is +6.

Solution

Chromite ore = FeCr2O4, where Fe is in +2 state (option A says +2, which is actually correct for Fe). But the statement says iron in +2, which is correct for chromite. However, option B about Cr being +6 in fusion product is the most definitively correct statement. In Na2CrO4, Cr is +6. CrO4²- is tetrahedral with all equivalent Cr-O bonds (option C is also correct). Cr(VI) is d⁰, so no unpaired electrons, yet it shows colour due to charge transfer (option D - this statement claims no unpaired electron AND it is coloured, which is true but the reason is charge transfer not d-d transition). Among the options, B is the most clearly and unambiguously correct.

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