$19\mathrm{g}$ of a mixture containing ${\mathrm{NaHCO}}_3$ and ${\mathrm{Na}}_2{\mathrm{CO}}_3$ on complete heating liberated $1.12\mathrm{L}$ of ${\mathrm{CO}}_2$ at STP. The weight of the remaining solid was $15.9\mathrm{g}$. What is the weight (in g) of ${\mathrm{Na}}_2{\mathrm{CO}}_3$ in the mixture before heating?