How many oxygen molecules are present in 113.97 liters of oxygen gas at STP?

At STP, one mole of any gas occupies 22.4 liters. This is called molar volume.

113.97 liters ÷ (22.4 L/mol) = 5.09 moles

Then convert moles to molecules (1 mole = 6.02 × 1023 molecules)

5.09 moles × (6.02 × 1023 molecules/mol) = 3.06 × 1024 molecules