Question

What is the oxidation number assigned to manganese in KMnO4?
(1) +7 (3) +3
(2) +2 (4) +4

Answer 1

Oxygen's oxidation number is -2 here, so we have -8.
Potassium is a group 1 element, therefore it has a oxidation number of +1. Therefore, Manganese's oxidation number must balance out +1 + -8 = -7. Therefore, Manganese's oxidation number is (1) +7

Answer 2

Let x be the oxidation of manganese.

We already know that the oxidation number of K is +1, of O is -2.

Because the compound is neutral, we have:
+1 +x + (-2)*4 = 0 (for we have 4 oxygens)

Now solve for x:
+1 +x -8 = 0
⇒ (+1 -8) +x = 0
⇒ -7 +x = 0
⇒ (-7 +7) +x = 0 +7
⇒ x = +7

The oxidation number of manganese in KMnO4 is +7.

Hope this helps~