Em tham khảo nha :
\(\begin{array}{l}
a)\\
{C_{{M_{HCl}}}} = 0,2M\\
{\rm{[}}{H^ + }{\rm{]}} = {C_{{M_{HCl}}}} = 0,2M\\
pH = - \log (0,2) = 0,699\\
b)\\
{C_{{M_{Ca{{(OH)}_2}}}}} = 0,4M\\
{\rm{[}}O{H^ - }{\rm{]}} = 2{C_{{M_{Ca{{(OH)}_2}}}}} = 0,8M\\
pOH = - \log (0,8) = 0,0969\\
pH = 14 - 0,0969 = 13,9031
\end{array}\)