Step-by-step explanation:
It is known that at STP, there are 22.4 L present in one mole of a substance.
Therefore, in 68.5 liters there will be 1 mol divided by 22.4 L times 68.5 L.
Mathematically,

= 3.05 mol
Hence, we can conclude that there are 3.05 moles present in 68.5 liters of oxygen gas at STP.