In the reaction 2 H2 + O2 -> 2 H2O, if you start with 4 moles H2 and 2 moles O2, how many moles of H2O can be formed?

• A. 2
• B. 4
• C. 6
• D. 8

Answer: (B)

Use the balanced equation to relate moles through the mole ratios. The reaction 2 H2 + O2 -> 2 H2O means 2 moles of hydrogen produce 2 moles of water using 1 mole of oxygen. To use all the oxygen you have (2 moles), you would need 4 moles of hydrogen, and you indeed have that amount. That means both reactants are consumed. The amount of water produced corresponds to the hydrogen used: 2 moles of H2 yield 2 moles of H2O, so 4 moles of H2 yield 4 moles of H2O. Therefore four moles of water can be formed.