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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

QUICK ANSWER…

B

SOLUTION…  

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

Option B

Now for the right answer to the above question:

1. Option A is incorrect.
2. Option B is correct.
3. C is incorrect.
4. D is not the correct answer.

QUICK TIPS…

You may please note these/this:

• Known oxidation number of other atoms are often used to derive that of the unknown as illustrated above.

Use the questions and answer section to correct, express, and contribute your views.

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/ culled from 2019 JAMB-UTME Chemistry question 40 /