You can find the formula of magnesium oxide by measuring the masses of magnesium and oxygen that combine. Weigh an empty crucible with its lid. Add a coil of clean magnesium ribbon and weigh again. The difference is the mass of magnesium. Heat the crucible strongly. Lift the lid a little every so often to let in air, but do not take it off, or white magnesium oxide smoke will escape. When the magnesium has stopped glowing, let the crucible cool and weigh it again.
Heat, cool and reweigh until the mass is constant. This shows the reaction is complete. The mass of oxygen that combined is the mass of magnesium oxide minus the mass of magnesium.
Example: 0.48 g of magnesium makes 0.80 g of magnesium oxide, so the mass of oxygen is 0.80 − 0.48 = 0.32 g. Mg: 0.48 ÷ 24 = 0.02. O: 0.32 ÷ 16 = 0.02. The ratio is 1 : 1, so the empirical formula is MgO.
If some of the smoke escapes, the mass of magnesium oxide recorded is too low.
