5,04 nhé!
\(n_{Ck}=\frac{7,56}{22,4}=0,3375\left(mol\right)=n_O\)
\(\Rightarrow m_O=5,4\left(g\right)\)
\(\Rightarrow m_R=18-5,4=12,6\left(g\right)\)
\(n_{H2}=\frac{5,04}{22,4}=0,225\left(mol\right)\)
\(2R+2xHCl\rightarrow2RCl_x+xH_2\)
\(n_R=\frac{0,45}{x}\left(mol\right)\)
\(\Rightarrow M_R=\frac{12,6x}{0,45}=28x\)
\(x=2\Rightarrow M_R=56\left(Fe\right)\)
\(n_{Fe}=0,225\left(mol\right)\)
\(n_{Fe}:n_O=0,225:0,3375=2:3\)
Vậy oxit sắt là Fe2O3