Question

The number of mole baco3 which contain 1.5 mole of oxygen atom is

Answer 1

moles of O atoms are present in 1 mole of BaCO3  

so 1 mole of O atoms will be present in 1/3 mole of BaCO3  

so 1.5 moles of O atoms will be present in 1/3 X 1.5 = 0.5 moles of BaCO3