nH2SO4= 0,04(mol)
PTHH: MgO + H2SO4 -> MgSO4+ H2O
x________x__________x(mol)
Al2O3 +3 H2SO4 -> Al2(SO4)3 + 3 H2O
y____3y_______y(mol)
Ta có: \(\left\{{}\begin{matrix}40x+102y=1,42\\x+3y=0,04\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=0,01\\y=0,01\end{matrix}\right.\)
-> mMgO=40x=0,4(g)
-> \(\%mMgO=\dfrac{0,4}{1,42}.100\approx28,169\%\\ \rightarrow\%mAl2O3\approx71,831\%\)