The sequence of coefficient that should be placed in the blank space to balance ii as follows
the sequence is 2 infront of Al 3 infront of O2 and 2 infront of Al2O3
that is 2Al +3O2 = 2Al2O3
from equation above 2 moles of Al reacted with 3 moles of O2 to form 2 moles of Al2O3
