KO2 First, determine how many moles of potassium you have by looking up the atomic weight. atomic weight of potassium = 39.0983 atomic weight of oxygen = 15.999 1.250 g / 39.0983 g/mol = 0.031970699 mol Now figure out the mass of oxygen by getting the difference between the product mass and the original mass of the potassium 2.273 g - 1.250 g = 1.023 g Moles of oxygen = 1.023 / 15.999 = 0.063941496 mol We now have a ratio of 0.031970699 : 0.063941496 for potassium and oxygen. We want to find a ratio of small integers that closely approximates that ratio. So divide all of the numbers by 0.031970699 (picked because it's the smallest value) and we get 1 : 2.000003 which is pretty darn close to 1 : 2. So the empirical formula for this potassium oxide is KO2