How many moles of magnesium phosphate, $M{g}_3{\left(P{O}_4\right)}_2$ , will contain 0.25 moles of oxygen atoms?

1 mol of  ${\text{mg}}_{\text{3}}{\left({\text{po}}_{\text{4}}\right)}_3$  contains -8 oxygen atoms
0.25 mole of oxygen contains  ${\text{mg}}_{\text{3}}{\left({\text{po}}_{\text{4}}\right)}_3$  is
$=\frac{0.25}{8}=3.125\times {10}^{-2}$