Oxygen in the form O2 has a molar mass of 31.9988 g mol^-1. In 4 grams there will be 4/31.9988 mol = 0.1250 mol. One mole of O2 equals 6.022 x 10^23 molecules of oxygen, and thereby 7.53 x 10^22 molecules of O2 in 4 gram. Since O2 is diatomic there are twice as many atoms as there are molecules and the final calculation to determine the number if atoms is 7.53 x 10^22 * 2 = 15.06 x 10^22 atoms of oxygen.
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