Question: how to calculate the percentage by mass of magnesium in magnesium oxide?
Answer: Magnesium atoms are heavier than oxygen atoms, so we expect more than 50% of magnesium in the weight composition. Taking just one atom of Mg and one atom of O you will get a mass of 16.0 + 24.3 = 40.3 for the unit that represents the minimum amount of magnesium oxide. 24.3/40.3 • 100 = 60.3% in mass of magnesium in it.