If the density of a certain gas at 30°C and 768 torr is 1.35 kg /m³, then density at STP is

We know, pV = nRT

$\mathrm{p}=\frac{\mathrm{n}}{\mathrm{V}}\mathrm{RT}=\frac{\mathrm{W}}{\mathrm{mV}}\mathrm{RT}=\frac{\mathrm{dRT}}{\mathrm{m}}$

$\text{ (where, }\mathrm{d}=\frac{\mathrm{W}}{\mathrm{V}}\text{ density in }{\mathrm{gL}}^{-1}\text{ ) }\mathrm{d}=\frac{\mathrm{pm}}{\mathrm{RT}}$

$\therefore \frac{{\mathrm{d}}_2}{{\mathrm{d}}_1}=\frac{{\mathrm{p}}_2{\mathrm{m}}_2{\mathrm{T}}_1}{{\mathrm{p}}_1{\mathrm{m}}_1{\mathrm{T}}_2}=\frac{{\mathrm{p}}_2{\mathrm{T}}_1}{{\mathrm{p}}_1{\mathrm{T}}_2} (\mathrm{for} \mathrm{a} \mathrm{given} \mathrm{gas})$

${\mathrm{d}}_2=\frac{1.35\times 760\times 303}{768\times 273}=1.48\mathrm{kg}/{\mathrm{m}}^3$