In the reaction 2 H2 + O2 → 2 H2O, with 3.0 mol H2 and 2.0 mol O2, what is the maximum amount of H2O that can form?

• A. 2.0 mol H2O.
• B. 3.0 mol H2O.
• C. 1.5 mol H2O.
• D. 4.0 mol H2O.

Answer: (B)

Limiting reagent determines how much product can form. In the equation 2 H2 + O2 → 2 H2O, 3.0 mol of H2 would require 1.5 mol of O2 to react completely (because 2 H2 need 1 O2). Since you have 2.0 mol O2, there’s more than enough O2, so hydrogen is used up first and is the limiting reactant. For this reaction, the amount of water produced equals the amount of hydrogen consumed because the stoichiometry links 2 H2 to 2 H2O. If all 3.0 mol of H2 react, you get 3.0 mol of H2O. Therefore, the maximum possible amount of water is 3.0 mol.