a, `⇒n_Al=(21,6)/(27)=0,8(mol)`
$PTHH: 4Al+3O_2-t^0->2Al2O3$
Theo PTHH: $4-------3------2(mol)$
Theo đề bài: $0,8------x------(mol)$
`⇒n_O_2=x=0,8*3:4=0,6mol`
`⇒V_O_2=0,6*22,4=13,44l`
b, `PTHH: 2KMnO4-t^0->MnO2+O2+K2MnO4`
Theo PTHH: $2------------1-------1-----1(mol)$
Theo đề bài: $x----------------------0,6-----(mol)$
`⇒n_{KMnO_4}=0,6*2:1=1,2(mol)`
`⇒m_{KMnO_4}=1,2*158=189,6g`