15.6 gram mixture of ${\mathrm{Na}}_2{\mathrm{CO}}_3$ and ${\mathrm{CaCO}}_3$ on strong heating  produced 2.2 grams of ${\mathrm{CO}}_2$ . Percentage of  sodium carbonate in the mixture is

 ${\mathrm{Na}}_2{\mathrm{CO}}_3$ cannot undergo thermal decomposition; All the ${\mathrm{CO}}_2$liberated is from ${\mathrm{CaCO}}_3$
$\therefore$$\begin{array}{l}{\mathrm{CaCO}}_3\left(\mathrm{s}\right)\to \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\\ 100\mathrm{g} 44\mathrm{g}\\ \text{'}\mathrm{X}\text{'} 2.2\mathrm{g}\end{array}$
 X = 5g
$\therefore {\mathrm{W}}_{{\mathrm{caco}}_3}=5\mathrm{g}$
$\begin{array}{l}{\mathrm{W}}_{{\mathrm{Na}}_2{\mathrm{CO}}_3}=\left(15.6-5\right)\mathrm{g}\\ =10.6\mathrm{g}\end{array}$
$\begin{array}{l}\mathrm{\%}\mathrm{of}{\mathrm{Na}}_2{\mathrm{CO}}_3\mathrm{in}\mathrm{the}\mathrm{initial}\mathrm{mixture}=\frac{10.6}{15.6}\times 100\\ =68\mathrm{\%}\end{array}$