Em tham khảo nha :
\(\begin{array}{l}
Ba{(OH)_2} \to B{a^{2 + }} + O{H^ - }\\
NaOH \to N{a^ + } + O{H^ - }\\
{\rm{[}}B{a^{2 + }}{\rm{]}} = {C_{{M_{Ba{{(OH)}_2}}}}} = 0,1M\\
{\rm{[}}N{a^ + }{\rm{]}} = {C_{{M_{NaOH}}}} = 0,2M\\
{\rm{[}}O{H^ - }{\rm{]}} = 2{C_{{M_{Ba{{(OH)}_2}}}}} + {C_{{M_{NaOH}}}} = 0,2 + 0,2 = 0,4M
\end{array}\)