What is Avogadro's law?

Avogadro's law states that at constant temperature and pressure, equal volumes of gas contain equal numbers of molecules: V₁/n₁ = V₂/n₂, where n is moles.

What is the molar volume of an ideal gas at STP?

22.4 L per mole at 0 °C and 1 atm. At SATP (25 °C, 1 bar) it is 24.79 L/mol.

Who proposed Avogadro's law?

Amedeo Avogadro proposed the underlying hypothesis in 1811. It became one of the cornerstones used by Clapeyron and others to assemble the Ideal Gas Law.

Why is Avogadro's law important?

It links the macroscopic volume of a gas to the number of molecules, allowing chemists to relate volumes of reactant and product gases directly to molar ratios in chemical equations.

Does Avogadro's law apply to real gases?

Approximately, in the same regime as the other simple gas laws: moderate temperatures and pressures. Deviations grow at high density.

How is Avogadro's law used in stoichiometry?

It converts gas volumes directly into mole ratios without needing mass or density. Because equal volumes of any gas at the same temperature and pressure hold equal numbers of molecules, the volume ratio of reacting gases equals their mole ratio in the balanced chemical equation.

Avogadro's Law Calculator

V₁/n₁ = V₂/n₂ at constant temperature and pressure.

Avogadro's Law Calculator

V₁/n₁ = V₂/n₂ (constant T,P). Enter 3 values.

V₁

n₁ (mol)

V₂ (solving for this)

n₂ (mol)

V₂

44.8 L

V₂ = V₁n₂/n₁ = 44.8 L

Avogadro's Law: Volume Per Mole

Avogadro's law states that at fixed temperature and pressure, the volume of a gas is directly proportional to the number of moles: V ∝ n, or V₁/n₁ = V₂/n₂. Add more gas to a flexible container at the same T and P and the volume grows proportionally.

Amedeo Avogadro proposed the underlying hypothesis in 1811: equal volumes of any gas, at the same temperature and pressure, contain equal numbers of molecules. This was a striking claim at the time, because it means the identity of the gas does not matter. One litre of hydrogen and one litre of carbon dioxide, at the same T and P, hold the same number of molecules, even though a mole of carbon dioxide weighs about 22 times as much as a mole of hydrogen.

Avogadro's Law Formula and Symbols

Symbol	Meaning	Unit
V₁	Initial volume	L, mL, m³, ft³
n₁	Initial amount of gas	moles (mol)
V₂	Final volume	L, mL, m³, ft³
n₂	Final amount of gas	moles (mol)

Molar Volume at STP

At Standard Temperature and Pressure (0 °C, 1 atm), one mole of an ideal gas occupies 22.4 L. At SATP (25 °C, 1 bar), the more common reference in modern chemistry, one mole occupies 24.79 L instead. This molar volume is a workhorse number in stoichiometry, letting chemists convert between gas volumes and moles in a chemical reaction without ever weighing anything.

Worked Example 1: Adding Gas to a Balloon

A balloon contains 1.0 mol of helium and has a volume of 22.4 L (STP). You add another 0.5 mol of helium. New volume at the same T and P: V₂ = V₁ × n₂/n₁ = 22.4 × 1.5/1.0 = 33.6 L. The volume grew by exactly 50%, the same percentage by which the amount of gas grew.

Worked Example 2: Reaction Volumes

2 H₂(g) + O₂(g) → 2 H₂O(g). By Avogadro's law, the volumes of reacting gases at the same T and P are in the same ratio as their moles, 2:1:2. Burning 4 L of H₂ requires 2 L of O₂ and produces 4 L of water vapour, all measured at the same temperature and pressure.

Connection to Ideal Gas Law

Solve PV = nRT for V at constant P and T: V = (RT/P) × n. The bracketed quantity is constant, recovering V ∝ n exactly. See the Ideal Gas Law calculator and the Combined Gas Law for related tools, or the Charles' law calculator for the temperature-volume relationship at constant pressure and moles.