100 ml of 0.1 M HCl and 100 ml of 0.1 M HOCN are mixed. What is the concentration of ${\text{"OCN}}^{-"};$ and pH of the solution? $(K_a=1.2\times {10}^{-6})$ 

$\text{HCl\hspace{0.17em}}\to {\text{\hspace{0.17em}H}}^{\text{+}}{\text{\hspace{0.17em}+\hspace{0.17em}Cl}}^{\text{-1}}\Rightarrow \text{\hspace{0.17em}0.1\hspace{0.17em}M\hspace{0.17em}×\hspace{0.17em}100\hspace{0.17em}=\hspace{0.17em}M\hspace{0.17em}×\hspace{0.17em}200\hspace{0.17em}}\Rightarrow \text{\hspace{0.17em}M\hspace{0.17em}=\hspace{0.17em}0.05M\hspace{0.17em}after\hspace{0.17em}dilution}$

$\text{HOCN\hspace{0.17em}}\rightleftharpoons {\text{\hspace{0.17em}H}}^{\text{+}}{\text{\hspace{0.17em}+\hspace{0.17em}OCN}}^{\text{-1}}\Rightarrow \text{\hspace{0.17em}0.1M\hspace{0.17em}×\hspace{0.17em}100\hspace{0.17em}=\hspace{0.17em}1M\hspace{0.17em}×\hspace{0.17em}200\hspace{0.17em}}\Rightarrow \text{\hspace{0.17em}M=\hspace{0.17em}0.05\hspace{0.17em}after\hspace{0.17em}dilurion}$

${\text{K}}_{\text{a}}\text{\hspace{0.17em}=\hspace{0.17em}}\frac{\left[{\text{H}}^{\text{+}}\right]\left[{\text{OCN}}^{-}\right]}{\left[\text{HOCN}\right]}{\text{\hspace{0.17em}1.2\hspace{0.17em}×\hspace{0.17em}10}}^{\text{-6}}\text{\hspace{0.17em}=\hspace{0.17em}}\frac{\text{0.05\hspace{0.17em}×\hspace{0.17em}0.05\hspace{0.17em}α}}{\text{0.05}}$

$\left[{\text{OCN}}^{\text{-1}}\right]{\text{\hspace{0.17em}=\hspace{0.17em}0.05\hspace{0.17em}α\hspace{0.17em}=\hspace{0.17em}1.2\hspace{0.17em}×\hspace{0.17em}10}}^{\text{-6}}$

$\left[{\text{H}}^{\text{+}}\right]\text{\hspace{0.17em}=\hspace{0.17em}}{\left[{\text{H}}^{\text{+}}\right]}_{\text{HCl}}\text{\hspace{0.17em}=\hspace{0.17em}0.05}$

$\text{pH\hspace{0.17em}=\hspace{0.17em}1.3}$