posted by bailey .
if not all the magnesium reacts, will the reported magnesium to oxigen ratio be too high or too low?
You could take the numbers from your experiment, decrease the mass of Mg a little and see which way the ratio goes. I think you will find that Mg:O ratio will be too high.
The mass of Mg you weigh to begin is some number. If some fails to react moles Mg will stay the same in your calculatins but mol Oxygen will be less. Mg to O then is larger than 1:1. It may be something like 1:0.8