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

One mole of ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$ contain 3 mol of Mg atoms, 2 mol of P atoms and 2 x 4 (= 8) mol of oxygen atoms

8 mol atoms of oxygen are present in

${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2=1\mathrm{mol}$

0.25 mol atoms of oxygen are present in

${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2=\frac{1}{8}\times 0.25=3.125\times {10}^{-2}\mathrm{mol}$.