Gay-Lussac's Law of Combining Volumes

Gay-Lussac's law of combining volumes says that at the same temperature and pressure, gases react in simple whole-number volume ratios. For example, 2 L of hydrogen and 1 L of oxygen make 2 L of water vapor, a 2:1:2 ratio. Avogadro's law means equal volumes hold equal numbers of molecules, so volume ratio equals molecule count and the equation coefficients. On this page, change the reaction and the volume to see how the gas volume ratios line up.

Volume Ratios in Gas Reactions

🎈 Intuition — Combining Balloons

①When gases react, their volumes form clean integer ratios

②H₂ 2L + O₂ 1L → H₂O vapor 2L (2:1:2)

③Gay-Lussac's law: gas-reaction volume ratios are simple integer ratios

Reaction0

📝 Reaction Notes

①#0: H₂ + Cl₂ → 2HCl (1:1:2)

②#1: 2H₂ + O₂ → 2H₂O (2:1:2)

③#2: N₂ + 3H₂ → 2NH₃ (1:3:2)

Avogadro's Law

🔬 Same Volume = Same Number of Molecules

①At the same temperature and pressure

②Regardless of the gas

③Equal volumes contain equal numbers of molecules

④That is Avogadro's law!

Volume22.4 L

Volume Ratio = Molecule Ratio

💡 The Key Link

①Thanks to Avogadro's law

②The gas-reaction volume ratio = molecule-number ratio

③In H₂ + Cl₂ → 2HCl

④Volume 1:1:2 = molecule 1:1:2

⑤Coefficients in the equation = volume ratio = molecule ratio

Summary

Gas Reaction Law

Gas reaction volume ratio = simple integer ratio

Volume ratio of reactants and products at the same T and P

Avogadro's Law

Same T, P → same V = same number of molecules

1 mol of any gas at 0 °C, 1 atm = 22.4 L

Key Relationship

Volume ratio = mol ratio = coefficient ratio

In gas reactions, volume, mol and coefficient ratios all match

🎯 Exam Points

①Gas reaction law: volume ratio = simple integer ratio

②Avogadro: same T,P → same V = same N

③In gas reactions, V ratio = coefficient ratio = mol ratio

④1 mol = 22.4 L at 0 °C, 1 atm

⑤Atom kinds and counts do not change in a reaction

Examples and Unit Test

Example 1

At the same temperature and pressure, hydrogen gas reacts with oxygen gas to make water vapor (2H₂ + O₂ → 2H₂O). For 20 mL of hydrogen to react fully, how many mL of oxygen are needed, and how many mL of water vapor form?

By the law of combining volumes, the volume ratio equals the coefficient ratio, so H₂ : O₂ : H₂O = 2 : 1 : 2.

For 20 mL of hydrogen, oxygen = 20 × (1/2) = 10 mL, and water vapor = 20 × (2/2) = 20 mL.

▸ 10 mL of oxygen needed / 20 mL of water vapor formed

In a gas reaction the volume ratio equals the coefficient ratio of the chemical equation.

Example 2

At the same temperature and pressure, how do the numbers of molecules compare in equal volumes of hydrogen and oxygen, and how does this connect to the volume ratio?

Avogadro's law: at the same temperature and pressure, equal volumes contain the same number of molecules regardless of the gas.

So in a gas reaction the volume ratio = the molecule-number ratio = the coefficient ratio, all the same.

▸ Equal volume = equal number of molecules (Avogadro) → volume ratio = molecule ratio = coefficient ratio

Thanks to Avogadro's law, counting volumes is the same as counting molecules.

Unit Test

Which statement about the law of combining volumes and Avogadro's law is correct?

①In a gas reaction the volume ratio is always a complicated decimal ratio

②Equal volumes of gas contain different numbers of molecules depending on the gas

③In a gas reaction the volume ratio equals the coefficient ratio of the chemical equation

④Avogadro's law holds only for solids

⑤At 0°C and 1 atm the volume of 1 mole of gas is 11.2 L

▸ ③ In a gas reaction the volume ratio equals the coefficient ratio of the chemical equation

The volume ratio of a gas reaction is a simple whole-number ratio (① wrong), and equal volumes of gas contain the same number of molecules regardless of the gas (② wrong, Avogadro).

Avogadro's law holds for gases (④ wrong), and at 0°C and 1 atm 1 mole of gas = 22.4 L (⑤ wrong); in a gas reaction the volume ratio = the coefficient ratio (③ correct).

← Previous

Definite Proportions

Ions & Electrolysis

Was this helpful? Support seegongsik