Group 1 element, forms an oxide in which its mass fraction is 83%. find this oxide.

If the mass fraction of an element is more than 50%, then this is an element with a mass of more than sixteen, you can start solving the problem from potassium, and then if this element is not suitable, take either sodium, or rubidium or cesium. Thus, potassium has a mass of 39 grams, oxygen is 16 grams, the compound has the formula K2O, which means that potassium is twice as much, therefore: 39 * 2 + 16 = 94 grams. 94 grams – 100%, and 78 grams (two potassium atoms) – X. Hence X = 78 * 100/94 = 82.98%. So we managed to pick up the element from the first time – this is potassium; the oxide of this element is K2O.