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Đáp án:

Câu 16: $C$

$7\times| \mathop{Mg}\limits^0\to \mathop{Mg}\limits^{+2}+2e$ 

$2\times\ 3\mathop{N}\limits^{+5}+ 7e \to 2\mathop{N}\limits^{+2}+ \mathop{N}\limits^{+4}$ 

$7Mg+20HNO_3\to 7Mg(NO_3)_2+4NO+2NO_2+10H_2O$ 

Câu 17: $B$

$n_{NO_2}=0,3$ mol 

Bảo toàn e: $2n_{Cu}=n_{NO_2}$ 

$\Rightarrow n_{Cu}=0,15$ mol 

Vậy $m=0,15.64=9,6g$