The balanced reaction is $2KClO_3 \rightarrow 2KCl + 3O_2$. 2 moles of $KClO_3$ produce 3 moles of $O_2$. So, 0.5 moles of $KClO_3$ will produce $(3/2) \times 0.5 = 0.75$ moles of $O_2$. At STP, 1 mole occupies 22.4 L. Volume = $0.75 \times 22.4 = 16.8$ L.