Đáp án đúng: B
75%.
$\begin{array}{l}\,\,\text{3Cu}\,\,\,\,\text{+}\,\,\,\,\text{8HN}{{\text{O}}_{\text{3}}}\xrightarrow{{}}\text{3Cu(N}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}\text{+}\,\text{2NO}\,\text{+}\,\text{4}{{\text{H}}_{\text{2}}}\text{O}\\\text{0,3}\,\text{mol}\,\,\,\,\,\,\text{0,8}\,\text{mol}\,\,\,\,\,\,\,\,\,\,\xleftarrow{{}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{0,2}\,\text{mol}\\\text{CuO}\,\text{+}\,\text{2HN}{{\text{O}}_{\text{3}}}\xrightarrow{{}}\text{Cu(N}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}}\text{+}\,{{\text{H}}_{\text{2}}}\text{O}\\\,\text{0,1}\,\,\leftarrow \,\,\text{0,2}\,\text{mol}\\\Rightarrow \text{H=}\frac{{{\text{n}}_{\text{Cu}}}}{{{\text{n}}_{\text{Cu}}}\text{+}{{\text{n}}_{\text{CuO}}}}\text{=}\frac{\text{0,3}}{\text{0,4}}\text{=75}{\scriptstyle{}^{\text{o}}\!\!\diagup\!\!{}_{\text{o}}\;}\end{array}$
