The answer is D. An oxygen molecule, O2 (because oxygen's natural state is diatomic), consists of two oxygen atoms. You would calculate the mass of a mole of oxygen molecules as 16.00 * 2 grams. To find the mass of one molecule, use the conversion factor 1 mole / 6.02e23 molecules. The mass of one oxygen molecule would then be 16.00 * 2 / 6.02e23