A glass tube of volume 112ml. containing a gas is partially evacuated till the pressure in it drops to $3.8\times {10}^{-5}$  torr at $0^{0}C$ . The number of molecules of the gas remaining in the tube is

        $n=\frac{PV}{RT}$

            $=\frac{\frac{3.8}{760}\times {10}^{-5}\times \frac{112}{1000}}{\frac{22.4}{273}\times 273}$

            $=\frac{1}{4}\times {10}^{-9}$

$\therefore$  number of molecules $=nN$

                                               $=\frac{1}{4}\times {10}^{-9}\times 6\times {10}^{23}$

                                               $=1.5\times {10}^{14}$