Question: how many molecules of o2 are there in 4.0 mol o2?
Answer: According to Avagadro, there are 6.022 × 10^23 molecules in 1 mole of any compound, irrespective of elements or the mass of compound. Thus in 3 moles of O2, there are 3 ×6.022× 10^23 molecules of O2, that is, 18.066×10^23 molecules of O2.