Question

Good evening. Thank you in advance for assisting me. Easy question but I don't know it. What does the -2 represent in the formula SO4-2

Answer 1

$SO4^{-2}$ is an anion of sulphuric acid.

$-2$ represents that net charge on sulphate ion is $-2$

And this negative charge is due to the formal negative charge of oxygen surrounding $SO4^{-2}$ ion.

Hence the oxidation state of $SO4^{-2}$ ion is $-2$.