Đáp án đúng: D
29,6%.
$\begin{array}{l}\left\{ \begin{array}{l}{{\text{n}}_{\text{CaC}{{\text{O}}_{\text{3}}}}}\,\text{+}\,{{\text{n}}_{\text{MgC}{{\text{O}}_{\text{3}}}}}\,\text{=}\,{{\text{n}}_{\text{C}{{\text{O}}_{\text{2}}}}}\,\text{=}\,\frac{\text{0,672}}{\text{22,4}}\,\text{=}\,\text{0,03}\,\text{mol}\\\text{100}{{\text{n}}_{\text{CaC}{{\text{O}}_{\text{3}}}}}\,\text{+}\,\text{84}{{\text{n}}_{\text{MgC}{{\text{O}}_{\text{3}}}}}\,\text{=}\,\text{2,84}\,\text{gam}\end{array} \right.\,\Rightarrow \left\{ \begin{array}{l}{{\text{n}}_{\text{CaC}{{\text{O}}_{\text{3}}}}}\,\text{=}\,\text{0,02}\\{{\text{n}}_{\text{MgC}{{\text{O}}_{\text{3}}}}}\,\text{=}\,\text{0,01}\end{array} \right.\\\Rightarrow \,{\scriptstyle{}^{\text{o}}\!\!\diagup\!\!{}_{\text{o}}\;}\text{MgC}{{\text{O}}_{\text{3}}}\,\text{=}\,\text{29,58}{\scriptstyle{}^{\text{o}}\!\!\diagup\!\!{}_{\text{o}}\;}\end{array}$
