Avogadro’s Law Calculator

How it works

V₁/n₁ = V₂/n₂ (constant T and P) · V₂ = V₁·n₂/n₁ · n₂ = n₁·V₂/V₁ · molar volume V/n = RT/P (22.414 L/mol at 0 °C and 1 atm; 24.465 L/mol at 25 °C and 1 atm)

Avogadro’s law states that, at the same temperature and pressure, equal volumes of gases contain equal numbers of molecules — so the volume of a gas is directly proportional to the amount in moles. It follows from the ideal gas law PV = nRT with P and T held constant, and the ratio V/n is the molar volume. It explains why balloons grow as you blow more air in and why gas reaction volumes come in whole-number ratios. Constants are from IUPAC and NIST; see OpenStax Chemistry 2e, section 9.2.

Worked example

A balloon holds 5 L of gas containing 0.2 mol. Adding gas until it holds 0.5 mol at the same temperature and pressure: V₂ = 5 × 0.5 / 0.2 = 12.500 L. The molar volume stays at 5 / 0.2 = 25.000 L/mol (1.115 × the STP value, consistent with room temperature), the volume grows by 7.500 L and 0.3 mol of gas was added.

Sources: OpenStax Chemistry 2e — Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law · NIST — molar volume of ideal gas

Frequently asked questions