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?

Answer 1

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.