$HCl \rightleftharpoons H+ + Cl-$ $1$
$\alpha \rightarrow \alpha \rightarrow \alpha $
$1-\alpha \rightarrow \alpha \rightarrow \alpha $
Có $K_a=\frac{\alpha ^2}{1-\alpha }=6,2.10^{-10}\Rightarrow \alpha \approx 2,5.10^{-5}$
$[H+]=\alpha \Rightarrow pH=-log[H+]=4,6$