Question: how to calculate empirical formula of metal oxide?
Answer: The ratio of atoms is the same as the ratio of moles. ... 
Mass of Mg = 0.297 g.
Mass of magnesium oxide = mass of Mg + mass of O.
0.493 g = 0.297 g + mass of O.
Mass of O = (0.493 – 0.297) g = 0.196 g.
There is 1 mol of Mg for 1 mol of O .