The density of a gas at STP is $2g/l$ . Its molecular weight is

                     $d=\frac{\text{molar}\text{mass}}{\text{molar}\text{volume}}$

$\text{molar}\text{mass}=d\times \text{molar}\text{volume}$

                        $=2\times 22.4$  

                        $=44.8g$               

       $\begin{array}{}\text{molecular}\text{wt}\text{is}44.8\end{array}$