Em tham khảo nha:
\(\begin{array}{l}
1)\\
a)\\
HCl + KOH \to KCl + {H_2}O\\
{n_{HCl}} = 0,2 \times 1 = 0,2\,mol\\
{n_{KOH}} = {n_{HCl}} = 0,2\,mol\\
{C_M}KOH = \dfrac{{0,2}}{{0,3}} = 0,67M\\
b)\\
{n_{KCl}} = {n_{HCl}} = 0,2\,mol\\
{V_{{\rm{dd}}}} = 200 + 300 = 500ml = 0,5l\\
{C_M}KCl = \dfrac{{0,2}}{{0,5}} = 0,4M\\
2)\\
Fe{(OH)_3} + 3HCl \to FeC{l_3} + 3{H_2}O\\
a)\\
{n_{HCl}} = 0,05 \times 1 = 0,05\,mol\\
{n_{Fe{{(OH)}_3}}} = \dfrac{{0,05}}{3} = \dfrac{1}{{60}}mol\\
{m_{Fe{{(OH)}_3}}} = \dfrac{1}{{60}} \times 107 = 1,78g\\
b)\\
{n_{FeC{l_3}}} = {n_{Fe{{(OH)}_3}}} = \dfrac{1}{{60}}mol\\
{C_M}FeC{l_3} = \dfrac{{\dfrac{1}{{60}}}}{{0,2}} = 0,083M\\
3)\\
{m_{KOH}} = 1200 \times 12\% = 144g\\
\text{ Gọi a là khối lượng KOH thêm vào }\\
\Rightarrow \dfrac{{144 + a}}{{1200 + a}} = 20\% \Leftrightarrow a = 120g
\end{array}\)