WBJEE · Chemistry · Electrochemistry

The formal potential of \(Fe^{3+}/Fe^{2+}\) in a sulphuric acid and phosphoric acid mixture \(\left(E^{\circ}=+0.61 \mathrm{V}\right)\) is much lower than the standard potential \(\left(E^{\circ}=+0.77 \mathrm{V}\right)\). This is due to

A formation of the species \(\left[\mathrm{FeHPO}_{4}\right]^{+}\)
B lowering of potential upon complexation
C formation of the species \(\left[\mathrm{FeSO}_{4}\right]^{+}\)
D high acidity of the medium

Verified Solution

Answer & Solution

Correct Answer

(D) high acidity of the medium

Step-by-step Solution

Detailed explanation

Since, \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\), both are strong acids so their presence makes the medium more acidic. The \(\mathrm{Fe}^{3+}\) ions form complex, IFeHPO \(\left._{4}\right]^{+}\) with these acid and hence, the concentration of…

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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