[treecreeper]

Hello

I know this is a really low level question for you guys, but bear with me! I'm a mature student at the grand old age of 31, and it's a looong time since I did chemistry in school. I've forgotten every last piece of it, so I'm starting from scratch.

I'm working my way through some practice tutorial questions as revision for a class test next week, and there some questions on oxidation numbers that I can't get my head around  . I have the answers to them, but I can't figure out why they are what they are.

Determine the oxidation number of the underlined element in each of the following:
a) VO₂⁺ (answer: +5)
b) Cr₂O₇^2- (answer: +6)
c) H₂SO₃ (answer: +4)

I know that the transition metals can have varying valencies, but without having the name of the compound written down with roman numerals, how are you supposed to work out (for example) that Vanadium is +5? If V can have up to 4 oxidation states, how can you tell which it will be?
And how can the S in c) be +4, if sulfite (SO₃^2-) has an overall valency of 2?

Sorry, I know that's really simple stuff probably. Any explanations of how this works would be very much appreciated!!

[Borek]

There are simple rules that tell you how to calculate oxidation numbers:

http://www.chembuddy.com/?left=balancing-stoichiometry&right=oxidation-numbers-method

Just remember oxidation numbers don't exist in reality, they are just an accounting device - a convenient one, making balancing redox reactions easier, but there is no real property of an atom in the molecule that can be measured to prove we are right about the oxidation number assigned.

[treecreeper]

Thanks Borek, I appreciate it