Question

How do you know the charge of common ions, i.e. nitrate, sulfate, etc?

Answer 1

Normally, the charge of common ions have to be committed to memory.

Explanation:

Sulfates, carbonates, biphosphates, all have `-2` charges. Halides all have `-1` charges. For metals, the charge derives from the Group number. And of course, the transition metals generally have a `M^(2+)` identity.

All these charges simply have to be learned. There are not that many, and often you are given a clue.