When 3.22 moles of #Al# reacts with 4.96 moles of #HBr#, how many or moles #H_2# are formed?
We need (i) a stoichiometrically balanced equation:
And thus we know that 1 equiv aluminum requires 3 equiv hydrogen bromide. But (ii) we do not have a stoichiometric hydrogen bromide in the starting conditions. Realistically, the only way we can do this is to ASSUME that all the hydrogen bromide oxidizes the aluminum quantitatively, and assume that there is no incomplete oxidation.