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Standard reduction potentials help judge whether an oxidizing agent is appropriate in a redox titration. The following half-reactions and their standard potentials are given: MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l) E° = 1.51 V Cr2O7^2-(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 7H2O(l) E° = 1.38 V Fe3+(aq) + e- → Fe2+(aq) E° = 0.77 V Cl2(g) + 2e- → 2Cl-(aq) E° = 1.40 V For the volumetric estimation of Fe(NO3)2, which one of the following statements is not correct?

1. MnO4- may be employed in aqueous HCl
2. Cr2O7^2- may be employed in aqueous HCl
3. MnO4- may be employed in aqueous H2SO4
4. Cr2O7^2- may be employed in aqueous H2SO4

Correct answer: Cr2O7^2- may be employed in aqueous HCl

Solution

The statement is incorrect because Cr2O7^2- is not stable in acidic solutions containing chloride ions, such as HCl, leading to the formation of chlorine gas, which interferes with the redox titration.

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