Gay-Lussac's Volume Puzzle

Why it matters

Mix two volumes of hydrogen with one volume of oxygen and they combine into two volumes of steam — a perfect 2 : 1 : 2 ratio of gases.

The idea in plain words

Observe the 2:1:2 ratio. Two balloons of H₂ pop with one of O₂ to form water. The volume ratio mirrors the stoichiometric coefficients.

Predict first

If 100 mL of carbon monoxide is burned completely (2CO + O₂ → 2CO₂), what volume of oxygen is required?

Gay-Lussac's Law: gases react in simple whole-number volume ratios. For 2CO + O₂ → 2CO₂, two volumes of CO need just one volume of O₂ and give two volumes of CO₂.

• 50 mL — correct
• 100 mL
• 200 mL
• 25 mL

What you do

Observe the 2 : 1 : 2 volume ratio: two volumes of H₂ react with one of O₂ to give two volumes of steam. Volume ratios hold only while everything is a gas.

Check yourself

In the reaction 2H₂ + O₂ → 2H₂O, what volume of O₂ is needed to react completely with 40 mL of H₂?

The ratio H₂:O₂ is 2:1. Therefore, 40 mL H₂ requires 40/2 = 20 mL O₂.

Why do gas volume ratios match mole ratios in these reactions?

Avogadro's Law states that at constant temperature and pressure, volume is directly proportional to the number of moles.

For CH₄ + 2O₂ → CO₂ + 2H₂O, if 10 mL of methane burns, what volume of CO₂ is produced?

The mole ratio of CH₄ to CO₂ is 1:1. Thus, 10 mL CH₄ produces 10 mL CO₂.

450 cm³ of carbon monoxide is ignited with 200 cm³ of oxygen (2CO + O₂ → 2CO₂). What is the composition of the final gas mixture?

O₂ is the limiting gas: 200 cm³ O₂ reacts with 2×200 = 400 cm³ CO to give 2×200 = 400 cm³ CO₂. Leftover CO = 450 − 400 = 50 cm³. So the final mixture is 400 cm³ CO₂ + 50 cm³ unused CO (no O₂ left).

• 20 mL — correct
• 40 mL
• 80 mL
• Because gases have no mass
• Avogadro's Law: equal volumes contain equal moles at same T and P — correct
• Because all gases are lighter than air
• 5 mL
• 10 mL — correct
• 20 mL
• 30 mL
• 400 cm³ CO₂ + 50 cm³ CO unused — correct
• 450 cm³ CO₂ only
• 400 cm³ CO₂ + 100 cm³ O₂ left
• 225 cm³ CO₂ + 225 cm³ CO