Calculate the percentage composition of carbon elements in ${\mathrm{Na}}_2{\mathrm{CO}}_3$.

Step 1: Molecular mass of Sodium carbonate ${\mathrm{Na}}_2{\mathrm{CO}}_3$ $=2\mathrm{x}23\mathrm{u}+1\mathrm{x}12\mathrm{u}+3\mathrm{x}16\mathrm{u}=106\mathrm{u}$

Step 2: Percentage composition of carbon = $$\begin{gathered} \frac{\mathrm{Mass}\mathrm{contributed}\mathrm{by}\mathrm{the}\mathrm{carbon}}{\mathrm{Total}\mathrm{molecular}\mathrm{mass}\mathrm{of}\mathrm{sodium}\mathrm{carbonate}}\times 100=\frac{12\mathrm{u}}{106\mathrm{u}}\times 100 \\ =11.32\% \end{gathered}$$

Hence, the percentage composition of carbon in ${\mathrm{Na}}_2{\mathrm{CO}}_3$ is $11.32\%$.