19 gms of a mixture containing $NaHCO_3\: and \: Na_2CO_3$ on complete heating liberated 1.12 lit of $CO_2$ at STP. The weight of the remaining solid was 15.9 gms. What is the weight ( in g ) of $Na_2CO_3$ in the mixture before heating ?

Question

$\begin {array} {1 1} (1)\;8.4 & \quad (2)\;15.9 \\ (3)\;4.0 & \quad (4)\;10.6 \end {array}$