Đáp án đúng: D
3,9 gam.
$\displaystyle \begin{array}{l}\left\{ \begin{array}{l}{{\text{n}}_{\text{Na}}}\,\text{+}\,\text{3}{{\text{n}}_{\text{Al}}}\,\text{=}\,\text{2}{{\text{n}}_{{{\text{H}}_{\text{2}}}}}\,\text{=}\,\text{0}\text{,5}\\{{\text{n}}_{\text{Na}}}\,\text{=}\,\text{2}{{\text{n}}_{\text{Al}}}\end{array} \right.\,\Rightarrow \,\left\{ \begin{array}{l}{{\text{n}}_{\text{Na}}}\,\text{=}\,\text{0}\text{,2}\\{{\text{n}}_{\text{Al}}}\,\text{=}\,\text{0}\text{,1}\end{array} \right.\\\Rightarrow \,{{\text{n}}_{\text{NaOH}}}\,\text{=}\,\,\text{0}\text{,1}\,\text{mol;}\,{{\text{n}}_{\text{NaAl(OH}{{\text{)}}_{\text{4}}}}}\,\text{=}\,\text{0}\text{,1}\,\text{mol}\\{{\text{H}}^{\text{+}}}\,\,\text{+}\,\text{O}{{\text{H}}^{\text{-}}}\,\xrightarrow{{}}\,{{\text{H}}_{\text{2}}}\text{O}\\\text{0}\text{,1}\,\leftarrow \,\text{0}\text{,1}\\\text{Al(OH)}_{\text{4}}^{\text{-}}\,\text{+}\,{{\text{H}}^{\text{+}}}\,\xrightarrow{{}}\,\text{Al(OH}{{\text{)}}_{\text{3}}}\downarrow \,\text{+}\,{{\text{H}}_{\text{2}}}\text{O}\\\,\,\,\text{0}\text{,05}\,\xleftarrow{{}}\,\text{0}\text{,05}\,\xrightarrow{{}}\,\text{0}\text{,05}\\\Rightarrow \,{{\text{m}}_{\downarrow }}\,\text{=}\,\text{3}\text{,9}\,\text{gam}\end{array}$
