Question

2Al+6HBr -> 2AlBr3+3H2
When 3.22 moles of Al reacts with 4.96 moles of HBr, how many moles of H2 are formed?

Answer 1

look at the mole ratio of the balanced reaction. It requires 6mol HBr for every 2mol Al

Now look at the mole ratio of what you're given. You have 4.96mol HBr and 3.22mol Al

\[\frac{6mol HBr}{2mol Al} \neq \frac{4.96mol HBr}{3.22mol Al}\]
One of the reactants will run out before the other. The two ratios SHOULD BE EQUAL, but they're not. One of the reactants on the left is too little.
Which reactant is there not enough of, according to the ratio on the left that would make the two ratios equal?