80 millimoles of ${\mathrm{MnO}}_4^{-}$ completely oxidises 30 millimoles of ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$ in basic medium. The concentration of 300 ml of ${\mathrm{SO}}_4^{-2}$ formed in the reaction is

$8{\mathrm{MnO}}_4^{-}+3{\mathrm{S}}_2{\mathrm{O}}_3^{-2}+{\mathrm{H}}_2\mathrm{O}\to 8{\mathrm{MnO}}_2+6{\mathrm{SO}}_4^{-2}+2{\mathrm{OH}}^{-}$

Number of milli moles of ${\mathrm{MnO}}_4^{-}$  = 80 

Number of milli moles of ${\mathrm{S}}_2{\mathrm{O}}_3^{-2}$ = 30

Number of milli moles of ${\mathrm{SO}}_4^{-2}$ = 60

60 = 300 ml x M

M = 0.2M