What volume of oxygen is needed at STP to completely react with 5.4g of aluminum?

Question

4Al + 3O2 --------> 2Al2O3

Answer 1

Determine moles of oxygen, then convert to volume using the fact that 1 mole of any gas at STP is 22.4 liters.

Note: the 3/4 term comes from the ratios of reactants in the balanced equation.

5.4g/27g/mol x (3/4) x 22.4L/mole = 3.36 liters

Answer 2

mole of Al = 5.4 / 27 = 0.2

mole of O2 = 3/4 * 0.2 = 0.15

volume of O2 = 0.15 * 22.4 = 3.36 L