In the reaction 2 H2 + O2 → 2 H2O, how many moles of H2 are needed per mole of O2?

In the reaction 2 H2 + O2 → 2 H2O, how many moles of H2 are needed per mole of O2?

• A. 2 moles of H2 per 1 mole of O2
• B. 1 mole of H2 per 2 moles of O2
• C. 3 moles of H2 per 1 mole of O2
• D. 1 mole of H2 per 1 mole of O2

Answer: (A)

In chemical equations, the numbers in front of the reactants show how many moles of each substance react relative to each other. The balanced equation 2 H2 + O2 → 2 H2O tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to form water. So for every mole of O2, you need 2 moles of H2. The other options don’t fit because they would imply different molar relationships than the 2:1 ratio shown by the equation (for example, 1:2 would use less hydrogen than required, 3:1 would demand three times as much hydrogen, and 1:1 would pair equal amounts).

In chemical equations, the numbers in front of the reactants show how many moles of each substance react relative to each other. The balanced equation 2 H2 + O2 → 2 H2O tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to form water. So for every mole of O2, you need 2 moles of H2. The other options don’t fit because they would imply different molar relationships than the 2:1 ratio shown by the equation (for example, 1:2 would use less hydrogen than required, 3:1 would demand three times as much hydrogen, and 1:1 would pair equal amounts).