From the equation 2 H2 + O2 -> 2 H2O, what is the mole ratio of H2 to H2O?

From the equation 2 H2 + O2 -> 2 H2O, what is the mole ratio of H2 to H2O?

• A. 1:1
• B. 1:2
• C. 2:1
• D. 3:1

Answer: (A)

The concept being tested is that the coefficients in a balanced chemical equation tell you the mole ratios between substances. In this reaction, the coefficients are 2 for H2 and 2 for H2O. That means for every 2 moles of hydrogen gas that react, 2 moles of water are produced. The mole ratio of H2 to H2O is therefore 2:2, which simplifies to 1:1. So, under complete reaction, the number of moles of H2 consumed equals the number of moles of H2O formed. If you used more hydrogen, you’d simply produce a proportional amount of water, maintaining the 1:1 relationship. Ratios like 1:2 would imply more water than hydrogen, and 2:1 or 3:1 would imply more hydrogen than water, neither of which matches the balanced equation.

The concept being tested is that the coefficients in a balanced chemical equation tell you the mole ratios between substances. In this reaction, the coefficients are 2 for H2 and 2 for H2O. That means for every 2 moles of hydrogen gas that react, 2 moles of water are produced. The mole ratio of H2 to H2O is therefore 2:2, which simplifies to 1:1. So, under complete reaction, the number of moles of H2 consumed equals the number of moles of H2O formed. If you used more hydrogen, you’d simply produce a proportional amount of water, maintaining the 1:1 relationship. Ratios like 1:2 would imply more water than hydrogen, and 2:1 or 3:1 would imply more hydrogen than water, neither of which matches the balanced equation.