CGPET 2011 Solved Paper

Coordination number is the number of ligands in the coordination sphere of the complex compound. Oxalate ion (C2O42−) is a bidentate ligand, hence coordination number of Cr in K3[Cr(C2O4)3] is 6 .
Let the oxidation number of Cr is x.
3+x+3(−2)=0
3+x−6=0
x−3=0
x=3