$2Fe_xO_y +(6x-2y)H_2SO_4\xrightarrow{} xFe_2(SO_4)_3+(3x-2y)SO_2 \uparrow +(6x-2y)H_2O$
$\begin{array}{*{20}{c}}
\begin{gathered}
\\\\\\\\\\\\\ \\
\end{gathered} \\
{(3x – 2y)}
\end{array}\left| \begin{gathered} 2\mathop {F{e_x}}\limits^{ + \frac{{2y}}{x}} – (6x – 4y)e \to x\mathop {Fe}\limits^{ + 3} {\,_2} \\ \mathop S\limits^{ + 6} + 2e \to \mathop S\limits^{ + 4} \\
\end{gathered} \right.$
Số mol $SO_2$ : $\frac{2,24}{22,4}=0,1 $ (mol)
Số mol $Fe_2(SO_4)_3$: $\frac{120}{400}=0,3 $ (mol)
Tỉ lệ mol : $\frac{n_{Fe_2(SO_4)_3}}{n_{SO_2}}=\frac{x}{3x-2y}=\frac{0,3}{0,1}=3 $
$\Rightarrow x=9x-6y \Rightarrow 6y=8x \Rightarrow \frac{x}{y}=\frac{6}{8}=\frac{3}{4} $
Công thức oxit sắt: $Fe_3O_4$