Question

Calculate the moles of SO2 that contains 3.8 moles of O atoms

Answer 1

SO₂ is made from 1 S atom and 2 O atoms. 
SO₂ → S + 2O

The stoichiometric ratio between SO₂ and O atom is 1 : 2.
Hence,
    moles of SO₂ = (moles of O atoms) / 2
                          = 3.8 mol / 2
                          = 1.9 mol

Hence, moles of SO₂ is 1.9 when there are 3.8 moles of O atoms.