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Question #44103

Difficulty: 8/10 | 🟢 Free

The Question

A 1.0 L1.0\text{ L} aqueous solution at 298 K298\text{ K} initially contains 0.10 mol0.10\text{ mol} of an amphiprotic salt \ceNaHA\ce{NaHA}. The acid dissociation constants of the diprotic acid \ceH2A\ce{H2A} are Ka1=10−3K_{a1} = 10^{-3} and Ka2=10−7K_{a2} = 10^{-7}. To this solution, 0.02 mol0.02\text{ mol} of \ceKMnO4\ce{KMnO4} and an excess of \ceH2O2\ce{H2O2} are added. The \ceKMnO4\ce{KMnO4} is completely reduced to \ceMn2+\ce{Mn^{2+}} by \ceH2O2\ce{H2O2}, which is oxidized to \ceO2(g)\ce{O2(g)}. Assuming no change in volume, that all \ceO2\ce{O2} escapes, and no \ceMnO2\ce{MnO2} is formed, what is the final pH of the solution?

(Given: log⁡2=0.30\log 2 = 0.30, log⁡3=0.48\log 3 = 0.48)

Options

  • A.

    7.18

  • B.

    5.00

  • C.

    7.60

  • D.

    6.82

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Amphiprotic Ion pH|KMnO₄ Redox pH Dependency|Successive Equilibrium Constants

Why This Question Is Tricky

Common Mistakes

This problem tests the coupling between redox stoichiometry and amphiprotic buffer equilibria.

The primary trap is assuming the pH remains constant at (pKa1+pKa2)/2=5.0(pK_{a1} + pK_{a2})/2 = 5.0 because \ceHA−\ce{HA-} is amphiprotic.

A secondary trap is failing to balance the overall redox reaction, mistakenly using the 8\ceH+8\ce{H+} consumed in the permanganate half-reaction and ignoring the 2\ceH+2\ce{H+} produced per \ceH2O2\ce{H2O2} oxidized, which would lead to an incorrect \ceH+\ce{H+} consumption of 0.16 mol0.16\text{ mol} instead of 0.06 mol0.06\text{ mol}.

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Question Information

SubjectChemistry
ChapterEquilibrium - Class 11 Part 1
ConceptAmphiprotic Ion pH
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree