The formula for determining the moles of a compound is grams (g) divided by molecular weight (MW), or moles = g / MW. The molecular formula of oxygen gas is O₂, and because each oxygen atom possesses an atomic weight of 16.00 grams per mole (g/mol) (this value is found on the Periodic Table), the MW of oxygen gas is 32.00 g/mol. Therefore, 32 g of oxygen gas represent (32 g) / (32.00 g/mol) = 1.0 moles of O₂.Source(s):
