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

 $1\text{ Mole of }{\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$ contains 8 mole of oxygen atoms  
$\therefore 8\text{ mole of oxygen atoms }\equiv 1\text{ mole of }{\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2\text{ mole of }{\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$
$$\begin{gathered} 0.25 \mathrm{moleofoxygenatom} \equiv \frac{1}{8}\times 0.25 \mathrm{moleof} {\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2 \\ =3.125\times {10}^{-2} \mathrm{moleof} {\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2 \end{gathered}$$