In part A, a student forgot to allow some air (oxygen) to get into the crucible while burning the magnesium. The oxygen insufficiency resulted in the formation of the magnesium nitride (Mg3N2) instead of the magnesium oxide. How will this error affect the magnesium-to-oxygen ratio and the empirical formula found?

Answer:

The chemical reaction may be written as

Mg(s) + N₂(g) + O₂(g) ---> MgO(s) + Mg₃N₂(s)

If you left some Mg₃N₂ in the crucible as product you would calculate a ratio of MgO to contain too little oxygen. When we weigh the final contents we wont know that the contents have more magnesium, we assume it’s pure MgO. Mg₃N₂ does not contain any oxygen, so there would be too little oxygen, but there is magnesium present so there is more magnesium than oxygen. The reported mole ratio of magnesium to oxygen would be too high, as some of the magnesium has yet to react, meaning that there will be more magnesium than there should have been. Since we did not completely burn all the magnesium and some un-burned magnesium remained, hence we will find that Mg:O ratio will be too high.

Instead of just having a one-to-one ratio of magnesium and oxygen atoms in the form of magnesium oxide, instead you have a higher proportion of magnesium resulting from the unreacted magnesium and the magnesium in the magnesium nitride. The empirical formula found to be not as MgO, instead it will be appear as Mg₂O.