5.6

a)
$c\left(HCl\right)=0{,}20\ \frac{mol}{dm^3}=\left[H_3O^+\right]$
$pH=-\lg\left[H_3O^+\right]=0{,}698$
b)
$c\left(HClO_4\right)=5{,}6\cdot10^{-3}\ \frac{mol}{dm^3}=\left[H_3O^+\right]$
$pH=2{,}251$
c)
$c\left(HNO_3\right)=0{,}025\ \frac{mol}{dm^3}=\left[H_3O^+\right]$
$pH=1{,}602$