nAl(OH)3=\(\dfrac{23.4}{78}\)=0.3(mol)
nNaOH=0.2*2=0.4(mol)
Al(OH)3+NaOH---->NaAlO2+2H2O
mol:\(\dfrac{0.3}{1}\)<\(\dfrac{0.4}{1}\) => NaOH dư
Al(OH)3+NaOH---->NaAlO2+2H2O
mol:0.3--->0.3--------->0.3 (mol)
mNaAlO2=82*0.3=24.6(g)
nNaOH dư=0.4-0.3=0.1(mol)
mNaOH thu được=40*0.1=4(g)