Thread: Past Paper Question: Sodium chlorate (VII)

Hello, to anyone reading this! I'm currently going through a chemistry past paper, and came across a question, asking me to suggest the formula to sodium chlorate (VII). I'm currently at As level chemistry and according to my teachers, we don't do anything related to oxidation powers in such contexts until' next year. So i was a bit off guard for the question. I had a go, and i came out with Na2Cl, as i took the (VII) part to be oxidation power, so i gathered that the compound had a overall charge of +1. So, i got onto the interwebs and searched for the actual formula and the theory to go with it. I found the formula to be NaClO4, and as for for theory, i got as far as the name that has the oxidation power after it, is the alternating number in the compound. I'm just confused with the theory... The compound can oxidise 7 electrons, but when i add up the oxidation numbers across the compound it equals: Na+1 , ClO-1 and O3-6. Added up equalling -6, so the compound has an oxidation of 6? I don't really get where the numbers are coming from. Any help is appreciated, thanks!

If the name suggests the oxidation state of ClO as +7 on Cl, the molecule would be NaClO4, if I'm not mistaken.

You just have to break down the molecule and apply the oxidation state rules (which I'll let you find).

Apply the rules to find the states of the other constituents. Remember that Cl must be the 7 charge as it won't likely be O due to vacancies in the d orbital. First row elements are +1, O is -2, so find Cl.

Na 1(+1) + Cl 1(X) + O 4(-2) = 0

Solve for X.

EDIT: Okay, I was trying to talk to someone while I was typing this so let me explain a little better.

You know from the name that you have a +7 charge on Chlorate. It can't be on the Oxygen atom because it doesn't have the free orbitals to accept the charge like Cl does. Right off the bat, you should know the charge you're solving for is Cl. You know from the rules that Na is +1 and O is -2 per atom. If you multiply the charge on the single Na atom by its oxidation number, you get +1. Since we are assuming the charge on Cl to be +7, you need to solve for O. You can do this by applying the rule that O is going to have a -2 charge per atom.

So, the REAL forumla you want is:

Na 1(+1) + Cl 7(+1) + O X(-2) = 0.

NOW solve for X.

Sorry about doing it backwards the first time.

So, Cl is in a (+7) state?

Originally Posted by JNickson

So, Cl is in a (+7) state?

Yes, that is what the (VII) after the molecule suggests. It denote the charge of the atom or part of the molecule preceding it. Fe(II)O suggests a +2 charge on the iron, for instance.

Thanks for the help!

No problem. If you need something explained more clearly, I'll try to do better.

Once you memorize the oxidation rules, you have a better toolset to solve these problems. For instance, if you don't know that O can be assumed as a -2 charge or that first column elements are +1, you don't even have a starting point.

I understand the rules of the groups and their ions. I just can't wrap my head around where this oxidation power is coming from, when the total charge of the compound is neutral? As your equation equals 0, i don't quite understand where the actual +7 oxidation is coming from

Well, getting the molecule to a neutral charge is the goal. If you know the rules, then you already know that Na, as a column I element, is +1 charge. You also know you only have 1 atom of Na. From the name of the molecule, we can derive that the +7 charge is on the Chlorine atome as Oxygen is assumed as -2 from our rules.

We have 2 known numbers (Na and Cl) and one variable (O). All we are trying to do is set the overall equation to 0 and solve for the Oxygen variable. That's just basic algebra. 1(1) + 1(-7) + (-2x) = 0

So, it isn't indicating that the overall has a oxidising power of 7, but rather that there is a element in the compound with the oxidation of 7?

Exactly. Since the number is not after sodium (ie, Sodium(VII)Chlorate) you know that it applies to the chlorate part of the molecule. You also know that Oxygen's charge is predetermined so only Chlorine can be +7.

Unless otherwise noted, you should assume an overall charge of 0.

Crystal clear now... Thanks a lot for the help, much appreciated ^^