How do i determine which valence is being used (balancing equations)

[supernova1203]

In balancing chemical equations, we are given let's say the reactants side of the equation and asked to find the products and balance if neccesary, if a metal(usually transition) has multiple valences, how do i know which of the valences is being used?

this has given me trouble to no end @_@

 

[supernova1203]

for instance look at this, Au has a valence of +1, +2 and +3 so how would i know beforehand which valence they are using? This equation is already balanced and the work is done for us, but in case it isnt, how do i determine that?

 

[supernova1203]

here we go

I know the polyatomic ions have only 1 valence so that's not the problem, the sulphate has a valence of 2


This is a website i got off someone on these forums a few days ago, it shows what i am talking about, some of them have several valences..

http://chemistry.about.com/library/weekly/aa122002a.htm


hope you can see the attachment i have linked in this thread.

 

[pmsrw3]

You know the valence by looking at the salts shown. For instance, in Au₂(SO₄)₃, the valence of the gold has to be +3 for each gold ion to balance the +2 of each of the 3 sulfate ions. The bromide salts are even easier.

 

[zaldar]

pmsrw is correct. If they are giving it to you as a free metal the valence is 0. Otherwise you look at the compound it is in.