For the reaction 2 H2 + O2 -> 2 H2O, if 4 moles H2 and 3 moles O2 react completely, how many moles of H2O are formed?

• A. 2 moles
• B. 3 moles
• C. 5 moles
• D. 4 moles

Answer: (D)

The main idea here is using the balanced equation to compare how much of each reactant is needed and what gets produced, which tells you the limiting reagent and how much product forms.

From the equation, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. This gives a hydrogen-to-water relationship of 1:1 when you scale to the amounts involved (for every 2 H2 that react, you get 2 H2O).

Check which reactant limits the reaction: if you use all four moles of hydrogen, you would need 2 moles of oxygen (because 2 H2 require 1 O2). You have 3 moles of oxygen, which is more than enough, so hydrogen is the limiting reactant. When the four moles of hydrogen are consumed, the same four moles of water are formed, since the reaction produces water in a 1:1 molar ratio with hydrogen under these conditions.

So the amount of water formed is four moles.