# How many moles of oxygen does 0.5 mol "CO"_2" contain?

$\text{0.5 mol CO"_2}$ contains $\text{1 mol O}$.
$\text{1 mol CO"_2}$ contains $\text{2 mol O}$.
$\therefore$ $\text{0.5 mol CO"_2}$ contains $\text{1 mol O}$.
$0.5 \cancel{\text{mol CO"_2xx(2"mol O")/(1cancel"mol CO"_2)="1 mol O}}$