Question

how magnesium phosphate gives 8moles oxygen

Answer 1

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1 mole of magnesium phosphate contains 8 moles of oxygen atoms ( as can be seen from chemical formula of magnesium phosphate there are 8 atoms of oxygen in one molecule of magnesium phosphate) . So, 1 mole contains 8 mole atoms, Implies, 0.25 moles contain 0.25*8 = 2 mole atoms of oxygen.

Answer 2

Formula of magnesium phosphate= Mg(PO4)2

since O=8
hence it gives 8 moles of oxygen