S atom is twice as heavy as an oxygen atom ( O is 16 and S is 32)
let's assume that 1 Arbitrary unit of sulfur is 1g heavy. this would mean that 2 units of Oxygen is 1g heavy(since it's half as heavy as S when comparing equal number of atoms). since we have 1.5g of oxygen, so there are 1.5*2=3 units of oxygen in that sample. and there are 1*1=1 units of Sulfur in that sample.
therefore the empirical formula is SO3
