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MCQ practice · SSC Chemistry Board questions · Chapter 7: Chemical Reactions

Chattogram Board 2025

What is the oxidation number of the central atom in K₂Cr₂O₇?

Think it through first, then pick an option.

  1. a - 6
  2. b + 3
  3. c + 5
  4. d + 6
Answer and explanation

Correct answer: d + 6

2(+1) + 2x + 7(−2) = 0, x = +6.

Why the other options are wrong

  • a - 6: Wrong sign: to balance K and O (net −12), Cr is positive.
  • b + 3: +3 does not balance: 2(+1) + 2(+3) − 14 ≠ 0.
  • c + 5: 2 + 2x − 14 = 0 gives x = +6, not +5.