Đáp án+Giải thích các bước giải:

 Câu 81:

\begin{align*}\left.\begin{aligned}3×\\2×\end{aligned}\right|&\left.\begin{aligned}S^{+6}+2e\rightarrow S^{+4}\\Fe^{0} \rightarrow Fe^{+3}+3e\end{aligned}\right.\\\end{align*} 

`2Fe + 6H_2SO_4 -> Fe_2(SO_4)_3 + 3SO_2 + 6H_2O`

`->` `a : b = 2 : 6 = 1 : 3`

`=>` `A`

Câu 82: 

\begin{align*}\left.\begin{aligned}3×\\2×\end{aligned}\right|&\left.\begin{aligned}S^{+6}+2e\rightarrow S^{+4}\\Al^{0} \rightarrow Al^{+3}+3e\end{aligned}\right.\\\end{align*} 

`2Al + 6H_2SO_4 -> Al_2(SO_4)_3 + 3SO_2 + 6H_2O`

`->` `a : b = 2 : 6 = 1: 3`

`=>` `C`

Câu 83:

\begin{align*}\left.\begin{aligned}1×\\1×\end{aligned}\right|&\left.\begin{aligned}N^{+5}+3e\rightarrow N^{+2}\\Al^{0} \rightarrow Al^{+3}+3e\end{aligned}\right.\\\end{align*} 

`Al + 4HNO_3 -> Al(NO_3)_3 + NO + 2H_2O`

`->` `a : b = 1 : 4`

`=>` `D`