Calculate the pH of a solution which contains 10 ml of 1 M HCl and 10 ml of 2 M NaOH

V_AN_A = 10x1=10 ;  V_BN_B = 10x2=20

 ${\mathrm{N}}_{\mathrm{mix}.}=\left[{\mathrm{OH}}^{-}\right]=\frac{{\mathrm{V}}_{\mathrm{B}}{\mathrm{N}}_{\mathrm{B}}-{\mathrm{V}}_{\mathrm{A}}{\mathrm{N}}_{\mathrm{A}}}{{\mathrm{V}}_{\mathrm{A}}+{\mathrm{V}}_{\mathrm{B}}}=\frac{20-10}{20}=0.5\mathrm{M}$

   $$\begin{gathered} \mathrm{pOH} = - \log {\mathrm{OH}}^{-} = - \log 0.5 \\ \mathrm{pOH} = 0.3 \\ \mathrm{pH} = 13.7 \end{gathered}$$