Equivalent weight of $Fe{S}_2[Mol.wt.=M]$ in the following reaction is $Fe{S}_2+O_2\to F{e}_2{O}_3+S{O}_2$

${\mathrm{Fe}}^{+2}{\mathrm{S}}_2^{-2}\to {\mathrm{Fe}}^{+3}{\mathrm{O}}^{-2}+{\mathrm{S}}^{+4}{\mathrm{O}}^{-2}$

Total change in Oxidation number of FeS₂ 

$= 1 (+1) + 2 (+5) = +11$

Equivalent weight of FeS₂ $=\frac{M}{11}$