Heating Mg in air converts it to MgO. By measuring mass before and after, we find the mass of oxygen that combined — giving us the empirical formula.
Lid lifting: The lid must be lifted periodically to let air in, but quickly replaced to prevent white MgO smoke from escaping. This is the key technique in this experiment.
Constant mass: Heating continues until two consecutive mass readings agree — confirming all Mg has reacted.
Common question: "Explain why the lid must be lifted during heating." → To allow oxygen in. If left sealed, reaction incomplete. If removed entirely, MgO smoke escapes → mass loss → formula error.
Uncertainty: mass of O gained is found by difference — propagate absolute uncertainties from both mass readings.
Expected ratio: Mg:O should be 1:1. Deviations indicate incomplete reaction (too few lid lifts) or MgO loss (lid off too long).
Lid off too long: MgO smoke escapes → final mass too low → O ratio appears less than 1.
Lid not lifted enough: Insufficient O₂ access → Mg unreacted → O ratio less than 1.
Mg₃N₂ formation: In limited O₂, Mg also reacts with N₂ → Mg₃N₂. Adding water and re-heating converts this to MgO + NH₃. IB expects you to know this side reaction.