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