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Cr\(_2\)O\(_7\)\(^{2-}\) + 6Fe\(^{2+}\) + 14H\(^+\) → 2Cr\(^{3+}\) +6Fe\(^{3+}\) + 7H\(_2\)O. In the above , the...

Chemistry
JAMB 1990

Cr\(_2\)O\(_7\)\(^{2-}\) + 6Fe\(^{2+}\) + 14H\(^+\) → 2Cr\(^{3+}\) +6Fe\(^{3+}\) + 7H\(_2\)O. In the above , the oxidation number of chromium changed from?

  • A. +7 to +3
  • B. +6 to +3
  • C. +5 to +3
  • D. -2 to +3
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Correct Answer: Option B
Explanation

The oxidation number of Cr in Cr\(_2\)O\(_7\)\(^{2-}\) is 

 Cr\(_2\)O\(_7\) = - 2

 2Cr + ( - 2) x 7 = - 2

2Cr - 14 = - 2

2Cr = - 2 + 14

2Cr = 12

\(\frac{2Cr}{2}\) = \(\frac{12}{2}\)

Cr = + 6

On the RHS, the oxidation number of Cr is +3.

Thus, the oxidation number changed from +6 to +3 - option B.


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