Mass fraction of oxygen in oxide E2O5 is 56.3%. Recognize element E.

The relative atomic mass of oxygen is 16. Find the molecular mass of oxygen in the unknown oxide:
16 * 5 = 80 g / mol;

Let’s find the molecular weight of the entire oxide:
80 / 0.563 = 142 g / mol;

Let’s find the molecular weight of the unknown element in the oxide:
142 – 80 = 62 g / mol;

Let’s find the relative atomic mass of element E:
62/2 = 31 (this is phosphorus);

Answer: The unknown element is phosphorus.