The number of moles of BaCO₃ , which contains 1.5 moles of oxygen atoms is

$\because$ 3 moles of oxygen are in 1 mole of BaCO₃
$\therefore$1.5 moles of oxygen are present in moles of BaCO₃
$=\frac{1}{3}\times 1.5=\frac{1}{2}=0.5$