27 g of Al will react completely with, how many grams of oxygen?

The balanced equation for the given reaction is:

4 Al+ 3 O₂ --------> 2Al₂O₃

Atomic mass of Al = 27g

Atomic mass of O₂ = 2 × 16 = 32g

Mole ratio of aluminium and oxygen can be given as:

Al    :  O₂

4     :   3

1    :  3/4

Mass of O₂ required = 3/4 × 32 =24g