The mass of $22.4\mathit{l}$ of gas at S.T.P. is equal to its gram molecular mass.
20 lit. of gas at S.T.P. weighs $=32\mathrm{g}$
$\therefore 22.4$ litres of gas will weigh $=\frac{32}{20}$ $\times 22.4$
$=35\cdot 84\mathrm{gms}.$
$\simeq 36\mathrm{gms}$ .
Therefore, Gram molecular weigh of the gas is 36 gms.