Đáp án đúng: C
1,2M hoặc 2,8M.
$\displaystyle \begin{array}{l}{{{m}}_{{A}{{{l}}_{{2}}}{{{(S}{{{O}}_{{4}}}{)}}_{{3}}}}}\,{=}\,\frac{{34}{,2}}{{342}}\,{=}\,{0}{,1}\,{mol;}\,{{{n}}_{{Al(OH}{{{)}}_{{3}}}}}\,{=}\,\frac{{7}{,8}}{{78}}\,{=}\,{0}{,1}\,{mol}\\{TH1:}\,{{{n}}_{{NaOH}}}\,{=}\,{3}{{{n}}_{{Al(OH}{{{)}}_{{3}}}}}\,{=}\,{0}{,3}\,{mol}\,\Rightarrow \,{{{C}}_{{M}}}\,{=}\,{1}{,2M}\\{TH2:}\,{{{n}}_{{Al(OH)}_{{4}}^{{-}}}}\,{=}\,{{{n}}_{{A}{{{l}}^{{3+}}}}}{-}{{{n}}_{{Al(OH}{{{)}}_{{3}}}}}\,{=}\,{0}{,1}\\\Rightarrow \,{{{n}}_{{NaOH}}}\,{=}\,{3}{{{n}}_{{Al(OH}{{{)}}_{{3}}}}}\,{+}\,{4}{{{n}}_{{Al(OH)}_{{4}}^{{-}}}}{=0}{,7}\,{mol}\,\Rightarrow \,{{{C}}_{{M}}}\,{=}\,{2}{,8M}\end{array}$
