Type II Ionic Compound help

[epicbattle]

Sorry if this is the wrong place to be posting this question. I'm new to the forums. I haven't had chemistry since Sophomore year of high school. So it's been about 8 years. I'm trying to brush up before I take my placement exam for college.

 

My question is that I can't quite understand how you find different cation values for metals. i.e. In trying to name Fe(N O3)3 it is iron (III) nitrate.

 

How do I know when it is supposed to change it's charge value? Sorry if this is a simple question. I do not have a teacher, I am trying to self learn all of basic chemistry.

 

Please feel free to recommend any helpful chem sites or forum pages.

 

Thanks.

[Physicsfan]

In reactions such as the reaction of iron with dilute acids, Fe 2+ salts are formed. but if an aqueous solution of an Fe 3+ salt undergoes a double decomposition reaction with an aqueous solution of another salt,(or with strong alkalis such as NaOH) the Fe 3+ salts are formed(with a different anion).