The mass of $22.4\mathit{l}$ of a gas at STP is equal to its gram molecular mass.
11.2 litres of gas at STP weighs $=24\mathrm{g}$
$\therefore 22.4$ litres of gas at STP weigh
$=\frac{24}{11.2}\times 22.4$
$=48\mathrm{gm}$