Đáp án:

 a) 36,5g

b) 19,5%

Giải thích các bước giải:

\(\begin{array}{l}
a)\\
mNaOH = C\%  \times m{\rm{dd}} = 200 \times 20\%  = 40g\\
nNaOH = \dfrac{m}{M} = \dfrac{{40}}{{40}} = 1\,mol\\
NaOH + HCl \to NaCl + {H_2}O\\
nHCl = nNaOH = 1\,mol\\
C\% HCl = \dfrac{{mct}}{{m{\rm{dd}}}} \times 100\%  = \dfrac{{1 \times 36,5}}{{100}} \times 100\%  = 36,5\% \\
b)\\
m{\rm{dd}}spu = 200 + 100 = 300g\\
nNaCl = nNaOH = 1\,mol\\
C\% NaCl = \dfrac{{1 \times 58,5}}{{300}} \times 100\%  = 19,5\% 
\end{array}\)

mình thì chỉ sơ sơ nên bạn tha khảo nha

mNaOH=200.20%=40(g)

=>nNaOH=1 mol

mdd sau pứ=200+100=300g

NaOH+HCl=>NaCl+H2O

1 mol=>1 mol=>1 mol

C%dd NaCl=58,5/300.100%19,5%

mHCl=36,5g

=>C%dd HCl=36,5/100.100%=36,5%