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Consider the equation below: Cr\(_2\)O\(_7 ^{2-}\) + 6Fe\(^{2+}\) + 14H\(^{+}\) \(\to\) 2Cr\(^{3+}\) + 6Fe\(^{3+}\) +...

Chemistry
JAMB 2019

Consider the equation below:

Cr\(_2\)O\(_7 ^{2-}\) + 6Fe\(^{2+}\) + 14H\(^{+}\) \(\to\) 2Cr\(^{3+}\) + 6Fe\(^{3+}\) + 7H\(_2\)O.

The oxidation number of chromium changes from

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

Cr\(_2\)O\(_7 ^{2-}\) + 6Fe\(^{2+}\) + 14H\(^{+}\) \(\to\) 2Cr\(^{3+}\) + 6Fe\(^{3+}\) + 7H\(_2\)O

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

Let the oxidation of Cr = x; 

2x + (-2 x 7) = -2 \(\implies\) 2x - 14 = -2

2x = 12 ; x = +6

Hence, the change in oxidation of Cr = +6 to +3

There is an explanation video available below.


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Explanation Video

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WAEC Past Questions, Objective & Theory, Study 100% offline, Download app now - 24709
WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
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WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
WAEC Past Questions, Objective & Theory, Study 100% offline, Download app now - 24709
Post-UTME Past Questions - Original materials are available here - Download PDF for your school of choice + 1 year SMS alerts
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