So if there is $M\; mole$ of $Mg_3\left(\mathrm{PO}_{4}\right)_{2}$ molecules then no of moles of oxygen atoms is $=\mathrm{M} \times 8$
According to question,
$M \times 8=0.25$
$\Rightarrow M=0.03125=3.125 \times 10^{-2}$
$I.\,SeO_3^{2 - } + BrO_3^ - + {H^ + } \to SeO_4^{2 - } + B{r_2} + {H_2}O$
$II.\,BrO_3^ - + AsO_2^ - + {H_2}O \to B{r^ - } + AsO_4^{3 - } + {H^ + }$