${\mathrm{S}}_2{\mathrm{O}}_3^{-2}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{SO}}_4^{-2}$. Equivalent weight of ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$ ( M = Molar mass)

O.S of sulphur atom in thiosulphate ion and sulphate ion is +2 and +6 respectively.

n-factor = 2(6-2) = 8 ;

Alternatively 

O.states of two sulphur atoms in thiosulphate ion are -2 and +6 whereas in sulphate ions both sulphur atoms have +6 state.

Thus change in O.S is -2$\to$+6 …change in O.S = 8 ;

n - factor = 8

Eq.wt =$\frac{\mathrm{M}}{8}$