Em tham khảo nha :

\(\begin{array}{l}
{C_M} = \dfrac{n}{V}\\
d = \dfrac{{{m_{{\rm{dd}}}}}}{v} \Rightarrow {m_{{\rm{dd}}}} = d \times v = d \times \frac{V}{{1000}}\\
V = 1000v\\
C\%  = \dfrac{m}{{{m_{{\rm{dd}}}}}} \times 100\%  = \dfrac{{n \times M}}{{{m_{{\rm{dd}}}}}} \times 100\% \\
 \Rightarrow C\%  = n \times M:d \times \dfrac{V}{{1000}} \times 100\% \\
 \Rightarrow C\%  = \dfrac{{{C_M} \times M}}{{d \times 10}}\\
 \Rightarrow {C_M} = \dfrac{{C\%  \times d \times 10}}{M}\\
 \Rightarrow {C_{{M_{NaOH}}}} = \dfrac{{20 \times 1,225 \times 10}}{{40}} = 6,125M
\end{array}\)

$C_{M}$ =C%.$\frac{10.D}{M}$ 

$C_{M[NaOH]}$ =C%.$\frac{10.D}{M}$ =20.$\frac{10.1,225}{40}$ =6,125 M