Em tham khảo nha:
\(\begin{array}{l}
4)\\
{m_{{\rm{dd}}MgC{l_2}}} \text{ sau pha trộn } = 20 + 80 = 100g\\
{m_{MgC{l_2}}} \text{ sau pha trộn }= 20 + 80 \times 10\% = 28g\\
{C_\% }MgC{l_2} = \dfrac{{28}}{{100}} \times 100 = 28\% \\
5)\\
{m_{{\rm{dd}}HCl}} \text{ sau pha trộn }= 50 + 450 = 500g\\
{m_{HCl}} = 450 \times 20\% = 90g\\
{C_\% }HCl = \dfrac{{90}}{{500}} \times 100\% = 18\%
\end{array}\)
