Weight of Magnesium phosphate that contains 4 moles of oxygen atoms in it is

Formula of Magnesium phosphate is ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$.

Every 1 mole of ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$contains 8 moles of Oxygen atoms.

In $\frac{1}{2}$ mole of ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$, there will be 4 moles of Oxygen atoms.

Mass of $\frac{1}{2}$ mole of Magnesium phosphate=$\frac{\mathrm{GMW} (262 \mathrm{g})}{2}=131 \mathrm{g} ;$