[govibe]

What's the difference between oxidation number and charge? which one do you use to figure out that cobalt(III) nitrate is Co(NO3)3?

Also, why can't Hydrogen sulfate (HSO4-) be named like an acid? Same with Hydrogen Phosphate (HPO4 2-)

[Yggdrasil]

In terms of ionic compounds (like cobalt (III) nitrate), the oxidation number of the metal is the same thing as its charge.  For covalent compounds, the oxidation number of an atom is not it's charge.  Conceptually, its a bit difficult to explain briefly, but it is basically the charge that the atom would carry if all the bonds in the covalent compound were ionic.

For the second question, the two species are ions which means that they are not named the same way as neutrally charged acids.