a.
\[4Al+3O_2\to 2Al_2O_3\]
b. \[n_{Al}=\dfrac{2,7}{27}=0,1(mol)\]
\[n_{O_2}=\dfrac{3}{4}\times 0,1=0,075(mol)\]
\[\to V_{O_2}=0,075\times 22,4=1,68(l)\]
c. \[n_{Al_2O_3}=\dfrac{1}{2}. n_{Al}=\dfrac 12. 0,1=0,05(mol)\]
\[\to m_{Al_2O_3}=102.0,05=5,1(g)\]