4 g of a mixture of Na₂CO₃ and NaHCO₃ on heating liberates 448 ml of CO₂ at STP. Percentage of Na₂CO₃ in the mixture is

Only NaHCO₃ undergo decomposition
$$\begin{gathered} \begin{array}{c}2{\mathrm{NaHCO}}_3⟶{\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2 \\ 168 \mathrm{g} 22,400 \mathrm{ml}\end{array} \end{gathered}$$
22400ml CO₂ obtained from 168 g
448 ml CO₂ produced from 
$\begin{array}{l}{\mathrm{WNaHCO}}_3=\frac{448\times 168}{22.400}=3.36\mathrm{g}\\ {\mathrm{WNaHCO}}_3=4-3.36=0.64\mathrm{g}\\ \mathrm{\%}{\mathrm{Na}}_2{\mathrm{CO}}_3=\frac{0.64}{4}\times 100=16\end{array}$