Consider the equation below:
The oxidation number of chromium changes from
- A. +5 to +3
- B. +6 to +3
- C. -2 to +3
- D. +7 to +3
From the options, it is obvious we don’t have a problem with the oxidation state of chromium on the right-hand side. Hence, lets focus on the left-hand side.
Oxidation state of chromium on the left-hand side of the equation =
2(Cr) + 7(O) = – 2
2(Cr) + 7(-2) = – 2
2(Cr) – 14 = – 2
2(Cr) = – 2 + 14
2(Cr) = 12
Therefore, Cr = 12/2 = +6
Now for the right answer to the above question:
- Option A is incorrect.
- Option B is correct.
- C is incorrect.
- D is not the correct answer.
You may please note these/this:
- Known oxidation number of other atoms are often used to derive that of the unknown as illustrated above.
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/ culled from 2019 JAMB-UTME Chemistry question 40 /