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1. The empirical formula is the mole ratio of magnesium to oxygen. If some of the magnesium fails to react with oxygen, you will end up with the same mass of magnesium but with a smaller than expected mass of oxygen, so the ratio of magnesium to oxygen will be larger. A large enough discrepancy could give a larger subscript for magnesium in the chemical formula.
2. Rules of nomenclature tell us that the empirical formula of magnesium oxide is MgO. Magnesium ion is the name for `Mg^2^+` and the oxide ion is `O^2^-` . The formula MgO has a net charge of zero.
3. If some of the magnesium reacts with nitrogen rather than oxygen there will be less than the expected amount of oxygen in the final product. This won't be visibly obvious, but since the molar mass of MgO is is 40.3 g/mole and the molar mass of Mg3N2 is 101 g/mole the mass will be different than if oxygen reacted with all of the Mg. For every mole or 101 g of Mg3N2 that formed, 3 moles or 120.9 g of MgO were prevented from forming. The product will have a smaller mass with Mg3N2 included. This would result in a lower ratio of oxygen to magnesium.
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