CUET · CHEMISTRY · PYQ PAPER 2023

In which case there is a change in oxidation number?

A Aqueous solution of \(\text{CrO}_4^{2-}\) is acidified.
B \(\text{SO}_2\) gas is passed through acidified \(\text{Cr}_2\text{O}_7^{2-}\) solution.
C \(\text{Cr}_2\text{O}_7^{2-}\) solution is made alkaline.
D \(\text{CrO}_2\text{Cl}_2\) is dissolved in \(\text{NaOH}\).

Verified Solution

Answer & Solution

Correct Answer

(B) \(\text{SO}_2\) gas is passed through acidified \(\text{Cr}_2\text{O}_7^{2-}\) solution.

Step-by-step Solution

Detailed explanation

Oxidation number of Cr in \( \text{Cr}_2\text{O}_7^{2-} \) = +6 Oxidation number of Cr in \( \text{Cr}^{3+} \) = +3 Oxidation number of S in \( \text{SO}_2 \) = +4 Oxidation number of S in \( \text{SO}_4^{2-} \) = +6 Change in oxidation number occurs.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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(B) \(C_{6}H_{5}O(CH_{2})_{3}CH_{3}\)	(II) Methyl phenyl ether
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