Oxygen gas reacts with powdered aluminum according to the following reaction:

Question

4Al(s)+3 O2(g) --> 2 Al2O3 (s)
How many liters of O2 gas, measured at 789 mm Hg and 35 C, are required to completely react with 2.2 mol of Al?

Answer 1

for every 4 mols of Al you need 3 mols of O2
so
mols O2 = (3/4)mols Al = .75 *2.2 = 1.65 mols

Now do P V = n R T with n = 1.65 mol