Deducing an empirical formula experimentally

Small masses of a substance, including magnesium, can be easily heated in a crucible. The crucible needs to be opened periodically to allow air (i.e. oxygen) to enter the apparatus. Lifting a crucible lid too often or for too long may lead to loss of product (i.e. lower final mass recorded). Using practical data and conservation of mass, chemists can calculate unknown reactant masses. Reactant masses can determine the empirical formula of a substance, including the ionic ratio of a metal and oxygen.