Molar mass of Mg3(PO4)2=260gmol−1 260gmol−1 of Mg3(PO4)2 contains 128g∕mol of oxygen atoms. 1 Mole of Mg3(PO4)2 contains ‌
128
16
moles of oxygen atoms 0.25×‌
16
128
mole of magnesium phosphate will contain 0.25 mole of oxygen atoms. or 3.125×10−2 moles of magnesium phosphate will contain 0.25 mole of oxygen atoms.