In which compound does chlorine have the highest oxidation number??
a) KCIO
b) KCIO2
c) KCIO3
d) KCIO4
I'm really stuck on these problem so please don't just tell me the answer i know its 4 but how do you get it, how do you find the oxidation number in situations like this?? thanks so much!
6 Answers

You just have to follow oxidation rules.
First rule that applies is that K (a group 1A metal) always has an oxidation state of +1
Second rule that applies is that O (in this type of compound) always has an oxidation state of 2
Third rule is that the oxidation state of a polyatomic must equal the charge of the polyatomic ion.
In hypochlorite (ClO) the sum of the oxidation states must equal 1. If Oxygen has a 2 state, Cl must have a +1 oxidation number.
In chlorite (ClO2), the sum also equals 1, so if O has a 2 and there are two of them for a total 4, Cl must have a +3 oxidation state.
You can see the oxidation state of Cl gets higher as more O is added, so with 4 oxygen KClO4 has the highest Cl oxidation state. 4 (2) = 8 oxygen, so Cl would be +7.
Hope that helps!

It's easy to find the oxidation number for Cl since you know the oxidation numbers for K and O. K is always +1 and O is always 2 (unless it's peroxide, which is 1).
a) KClO . . .in any compound, the sum of the oxidation numbers must be zero.
So, ox. no. K + ox. no. Cl + ox. no. O = 0
. . . . .+1 . . .+ ox. no. Cl + . . .2 . . = 0
ox. no. Cl = +1
b) KClO2 . . .here we have 2 O, so the ox. no. of 2 O = 2 x 2 = 4
+1 + ox. no. Cl + (4) = 0
ox. no. Cl = +3
For c) and d), the oxidation numbers for Cl are 5 and 7, respectively.

d, KClO4.
What you do is look at the common oxidation numbers for the other elements in the compound. In this case, K is always +1, and O is usually 2. From there, you can work out what oxidation number Cl should have  it is +7 in the case of KClO4, and that's the highest oxidation number Cl can attain.

In (d) the K is +1, and each O is 2, the without the Cl we have 7. So the Cl must be +7 to make the whole compound zero.

So would this answer not change if the answer choices were NaClO, NaClO2, NaClO3, and NaClO4?

(mass of O)(# of O) divided by (molecular formula)
answer is d