nNa=0.2(mol);nAlCl3=0.06(mol)
Na + H2O --> NaOH + 1/2 H2
3NaOH + AlCl3 --> Al(OH)3 + 3NaCl
..0.18........0.06..........0.06..................(mol)
NaOH + Al(OH)3 --> NaAlO2 + 2H2O
0.02..........0.02.....................................(mol)
=> nAl(OH)3 dư=0.04(mol)
Bảo toàn Al => nAl2O3=0.02(mol)
=> mAl2O3=2.04(g)