$28{\mathrm{NO}}_3^{-}+3{\mathrm{As}}_2{\mathrm{S}}_3+4{\mathrm{H}}_2\mathrm{O}\stackrel{ }{\to }6{\mathrm{AsO}}_4^{3-}+28\mathrm{NO}+9{\mathrm{SO}}_4^{2-}+8{\mathrm{H}}^{+}.$

What will be the equivalent mass of ${\mathrm{As}}_2{\mathrm{S}}_3$ in above reaction : (Molecular mass of ${\mathrm{As}}_2{\mathrm{S}}_3$ = M)?

From the given equation ,

                                  ${\stackrel{+3}{\mathrm{As}}}_2{\stackrel{-2}{\mathrm{S}}}_3\to \stackrel{+5}{\mathrm{As}}{\mathrm{O}}_4^{-3} +\stackrel{+6}{ \mathrm{S}}{\mathrm{O}}_4^{-2}$

 Change in oxidation state of  ‘As’  = 2(2) =4

 Change in oxidation state of  ‘S’ = 3(8) = 24

Total change in oxidation state = n-factor = 24+4=28

$\therefore$ Equivalent weight of ${\mathrm{As}}_2{\mathrm{S}}_3 = \frac{\mathrm{M}}{28}$

Where M= molecular mass