How many moles magnesium phosphate, ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$ will contain 0.25 mole of oxygen atoms ?

 $8\mathrm{moles} \mathrm{of}\mathrm{Oxygen} \mathrm{present} \mathrm{in} 1\mathrm{mole}{\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$

$\Rightarrow 0.25\mathrm{mole}\equiv \frac{1}{8}\times 0.25$

                               $=3.125\times {10}^{-2}\mathrm{moles}$