Question
Balance the following equation by oxidation number method:
(in alkaline medium)
Solution
(i) The skeleton equation along with oxidation number of each atom is
(ii) Noting the changein O.N. of above atoms,
Increase S = +4 to +6 i..e 2
Decrease Cr = +6 to +3 i.e. 3
(iii) Equalise the increase/decrease in O.N. by
multiplying 
(iv) Balance Cr atoms by multiplying [Cr(OH)4]- by 2 and balance S atoms by multiplying (SO4)2- by 3.
(v) Balance O atoms by adding three H2O molecules on the reactant side.
(vi) Balance the H atoms
Net charge on LHS = -4 - 6 = -10
Net charge on RHS = -2 -6 = -8
adding two OH-(basic medium) on RHS and adding 2 water molecules on the reactant side.
