A solute may be dissolved in a solvent because it ionizes in solution or because it dissociates in solution. For instance, when iron metal is dissolved in acid, it is oxidized to FeIII. The chemical identity of the solute has been changed. Similarly, many substances dissociate in solution hence changing the original chemical identity of the solute.
74.62 g of magnesium oxide is formed from 45.00 g magnesium so 74.62-45.00= 29.62 g of oxygen is consumed or in other words a new compound is formed in the burning of magnesium in oxygen with a heavier mass than the pure magnesium.
