$$\begin{gathered} {\mathrm{AgNO}}_3+\mathrm{NaCl}\to \mathrm{AgCl}+{\mathrm{NaNO}}_3 \\ 1\mathrm{mole} 1\mathrm{mole} 1 \mathrm{mole} \end{gathered}$$

$\begin{array}{l}0.1\mathrm{M}{\mathrm{AgNO}}_3\mathrm{will}\mathrm{react}\mathrm{with}0.1\mathrm{M}\mathrm{NaCl}\mathrm{to}\mathrm{form}0.1\mathrm{M}{\mathrm{NaNO}}_3.\\ \hspace{0.17em}\mathrm{conc}.\mathrm{of} {\mathrm{NO}}_3^{-}=\frac{0.1}{2}=0.05\mathrm{M}\\ (\mathrm{since} \hspace{0.17em}{\mathrm{NaNO}}_3\to {\mathrm{Na}}^{+}+{\mathrm{NO}}_3^{-})\end{array}$