\(n_{Fe}=\dfrac{11.2}{56}=0.2\left(mol\right),n_{Mg}=\dfrac{2.4}{24}=0.1\left(mol\right)\)
\(Fe+H_2SO_4\rightarrow FeSO_4+H_2\)
\(Mg+H_2SO_4\rightarrow MgSO_4+H_2\)
\(FeSO_4+2NaOH\rightarrow Fe\left(OH\right)_2+Na_2SO_4\)
\(MgSO_4+2NaOH\rightarrow Mg\left(OH\right)_2+Na_2SO_4\)
\(4Fe\left(OH\right)_2+O_2\underrightarrow{^{t^0}}2Fe_2O_3+4H_2O\)
\(Mg\left(OH\right)_2\underrightarrow{^{t^0}}MgO+H_2O\)
\(m_{cr}=m_{Fe_2O_3}+m_{MgO}=\dfrac{0.2}{2}\cdot160+0.1\cdot40=20\left(g\right)\)
