Dichromate ion is treated with base, the oxidation number of Cr in the product formed is ___________.

Solution

Chemistry ma’am · 4 steps

Base only turns orange dichromate into yellow chromate, so chromium stays at +6.

1. What base does. Hydroxide ions convert dichromate into chromate. This is an acid–base equilibrium, not a redox reaction.

   Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O

2. Find Cr in chromate. Let the oxidation number of Cr be x. Each O is -2, so in CrO₄²⁻, x + 4(-2) = -2, which gives x = +6.
3. Check against dichromate. In Cr₂O₇²⁻, 2x + 7(-2) = -2, so x = +6 there too. The oxidation number does not change, as expected for a reaction that is not redox.
4. So. Chromium in the product CrO₄²⁻ has an oxidation number of 6.

The common slip: 3. Treating OH⁻ as a reducing agent and writing Cr(OH)₃ as the product gives 3. Hydroxide only shifts the dichromate–chromate equilibrium, so Cr stays at +6.

The idea to keep. Dichromate and chromate interconvert just by changing the pH, and Cr is +6 in both. A colour change from orange to yellow does not always mean a redox change.

Answer from the NTA final answer key. Solution by PrepWiser.