What is the oxidation state of S in $\mathrm{SO}_4^{2-}$ ?

What is the general molecular formula of aldehydes?

Which among the following is NOT a pair of isomers?

Identify the total number of complexes having bidentate ligands in them from following list of complexes.

a) Tetracyanonickelate(II) ion

b) Trioxalatocobaltate(III) ion

c) Sodium hexafluoroaluminate(III)

d) bis(ethylenediamine)dithiocyanatoplatinum(IV)