The density of air at STP is 0.00130 g/ml. Thus, vapour density of air will be

$$\begin{gathered} \mathrm{p}=\frac{\mathrm{dRT}}{\mathrm{m}} \\ \therefore \mathrm{m}=\frac{\mathrm{dRT}}{\mathrm{p}} \\ \mathrm{d}=0.0013\mathrm{g}/\mathrm{mL}=1.3\mathrm{g}/\mathrm{L} \\ \therefore \mathrm{m}=\frac{1.3\times 0.0821\times 273}{1}=29.12\mathrm{g}{\mathrm{mol}}^{-1} \\ \therefore \text{ Vapour density }=\frac{\text{ molar mass }}{2}=\frac{29.12}{2}=14.56 \end{gathered}$$