The mass of $22.4\,\mathrm{l}$ 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.