To calculate the number of grams in 3.13 x 10^26 molecules of oxygen, you need to first calculate the molar mass of oxygen, which is 32.00 grams per mole. Once you have the molar mass, you can then use the Avogadro's number (6.02 x 10^23 molecules per mole) to calculate the number of grams:
3.13 x 10^26 molecules O2 x (32.00 grams/6.02 x 10^23 molecules) = 0.0524 grams O2