Đáp án:

\(\begin{array}{l}
a)\\
C{\% _{NaOH}} = 16,67\% \\
b)\\
{C_{{M_{NaOH}}}} = 5M
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
a)\\
C{\% _{NaOH}} = \dfrac{{{m_{NaOH}}}}{{{m_{NaOH}} + {m_{{H_2}O}}}} \times 100\%  = \dfrac{{20}}{{20 + 100}} \times 100\%  = 16,67\% \\
b)\\
{n_{NaOH}} = \dfrac{m}{M} = \dfrac{{20}}{{40}} = 0,5mol\\
{V_{{H_2}O}} = 100ml = 0,1l\\
{C_{{M_{NaOH}}}} = \dfrac{n}{V} = \dfrac{{0,5}}{{0,1}} = 5M\\

\end{array}\)

Bạn tham khảo nha!

a. `-` `m_{dd} = 20 + 120 = 120` `(g)`

`->` `C% =` $\dfrac{20}{120}$ ` × 100% = 16,67%`

b. `-` `100` `(ml)` `= 0,1` `(l)`

`-` $n_{NaOH}$ `=` $\dfrac{20}{40}$ `= 0,5` `(mol)` 

`->` `C_M =` $\dfrac{0,5}{0,1}$ `= 5` `(M)`