Question: how many molecules are present in 1 litre of oxygen?
Answer: The question should have been how many molecules of oxygen are present in 1 litre of air containing 80% of O2. 1 mole of any gas at STp, occupies 22.4 litre. 22400ml of O2 gas contains 6.022x 10^23 molecules. =2.15 x 10^22 molecules.