In a hurry Simon did not allow magnesium to react. Will his reported magnesium to oxygen ratio be given as too high or too low? And why?
Let's make up an example to see what well happen.
If we do the experiment correctly, we would get this. Starting with 24.3 g Mg we will get 40.3 g of the oxide which means 40.3 g oxide - 24.3 g Mg = 16 oxygen reacted.
mols Mg = 24.3/24.3 = 1 and mols oxygen atoms = 15/16 = 1 so the formula is MgO.
Suppose we were in a hurry like Simon and we didn't let all of the Mg react. So less Mg reacted (but we still think all 24.3 g reacted) which means less than 16 g oxygen reacted and that means the final mass of the oxide is say 32.3. Mass oxygen = 32.3-24.3 = 8,
mols Mg = 24.3/24.3 = 1 and mols oxygen atoms = 8/16 = 0.5 so formula is Mg20.