Solving moles-to-moles limiting reactant

Question

Hydrogen (H2) gas and oxygen (O2) gas react to form water (H2O) vapor. Suppose you have 3.0 mol of H2 and 11.0 mol of O2 in a reactor.

Suppose as much as possible of the H2 reacts. How much will be left? Round your answer to the nearest 0.1 mol.

Answer 1

First write the balanced equation: 2H2 + O2 ---- > 2 H2O.

Since H2 reacts with O2 in a 2 to 1 mole ratio, and you begin with 3.0 moles of H2, that means you would need at least 1.5 moles of O2 for a complete reaction. With 11 moles of O2, there is more than enough O2 for a complete reaction of H2.

So, there will no H2 left after the reaction, but there will be some leftover O2. Since all 3 moles of H2 reacted, this would use up 1.5 moles of O2. As there were 11.0 moles of O2 to begin with, there would be 11.0 – 1.5 = 9.5 moles of O2 left over.