What is the molecular weight of a gas whose density at ${40}^{0}C$ and 785 mm of Hg pressure is 1.3 g ${\mathrm{L}}^{-1}?$

We know that PV = nRT

$\begin{array}{l}\mathrm{PV}=\frac{\mathrm{w}}{\mathrm{M}}\mathrm{RT}\mathrm{or}\mathrm{P}=\frac{\mathrm{w}}{\mathrm{V}}\times \frac{1}{\mathrm{M}}\mathrm{RT}\\ \mathrm{Or}\mathrm{P}=\frac{\mathrm{d}}{\mathrm{M}}\mathrm{RT}\\ \mathrm{Given}, \mathrm{d}=1.3\mathrm{g}/\mathrm{ml} , \mathrm{P}=785\mathrm{mm} \mathrm{of} \mathrm{Hg}=\frac{785}{760}\mathrm{atm}, \mathrm{T}=40°\mathrm{C}+273=313\mathrm{K}\\ \mathrm{Now},\frac{785}{760}=\frac{1.3}{\mathrm{M}}\times 0.0821\times 313\mathrm{or}\mathrm{M}=32.4\mathrm{or}32\mathrm{g}\end{array}$