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JAMB Chemistry 1990 Question 24

Question 24

Redox Reactions

Cr2O72− + 6Fe2+ + 14H+ → 2Cr3+ + 6Fe3+ + 7H2O. In the above equation, the oxidation number of chromium changed from

  1. A.+7 to +3
  2. B.+6 to +3
  3. C.+5 to +3
  4. D.−2 to +3.
Show the answer and explanation

Answer: B (+6 to +3)

Explanation

In Cr2O72−: 2x + 7(−2) = −2, so x = +6. It ends as Cr3+, so it goes from +6 to +3.