8. What is the oxidation number of N in KNO3?

The oxidation state of N in the KNO3 is +5

Explanation:

Oxidation rules:

1. Oxygen is -2, unless in peroxides.

2. Group 1 metals = +1

3. Group 2 metals = +2

4. If the molecule is neutral, all of the oxidation numbers have to add up to zero.

5. If the molecule is charged, all of the oxidation numbers have to add up to the charge of the molecule.  

So, the given formula represents the salt compound formula unit of potassium nitrate:  KNO3  

The formula unit is uncharged.

From our rules, we know that,

O = -2

And we can find K on the periodic table, in the first group, thus giving it a +1 charge. Now let's put it all together.

K = +1

N = x

O = -2

Let's take into account the number of atoms of each element we have and make an equation since we know everything has to add up to zero since the molecules are neutral.

+1 +x+3 (-2) = 0 (notice we multiplied 3 by -2 because in the formula we have 3 atoms of oxygen with -2 charge each)

x - 5 = 0

x = 5

Therefore, the oxidation number of N in KNO3 is +5.