# Gas Mixture Molar Mass Calculator

URL: https://hifreetools.com/gas/mixture

> List the gases in a mixture with their mole (volume) percentages or mass percentages, and get the mixture's average molar mass plus the other set of fractions. Useful for welding and calibration gas blends, air and process gas specifications.

## Quick answer

A mixture's molar mass is the mole-fraction-weighted average: M = Σ xᵢ·Mᵢ. Dry air (78.08% N₂, 20.95% O₂, 0.93% Ar, 0.04% CO₂) is 28.966 g/mol. To turn mole % into mass %, multiply each mole fraction by its molar mass and divide by M: 5 mol% hydrogen in nitrogen is only 0.377 mass%.

## Formula

- M = Σ xᵢ·Mᵢ wᵢ = xᵢ·Mᵢ / M xᵢ = (wᵢ/Mᵢ) / Σ(wⱼ/Mⱼ)

## Examples

- Forming gas, 5% hydrogen in nitrogen: 5 mol% H₂ + 95 mol% N₂ → M = 26.714 g/mol; H₂ = 0.377 mass%
- Dry air: 78.08% N₂, 20.95% O₂, 0.93% Ar, 0.04% CO₂ (mole) → M = 28.966 g/mol; O₂ = 23.14 mass%
- A blend specified by mass: 50 mass% CO₂ + 50 mass% Ar → M = 41.880 g/mol; 47.58 mol% CO₂, 52.42 mol% Ar

## FAQ

**What is the molar mass of air?**
28.966 g/mol for dry air (78.08% N₂, 20.95% O₂, 0.93% Ar, 0.04% CO₂ by mole).

**Is mole percent the same as volume percent?**
For ideal gases, yes: equal volumes at the same temperature and pressure hold equal moles. Gas certificates often say vol% or mol% interchangeably.

**How do I convert mol% to mass%?**
Multiply each mole fraction by its molar mass, add them up to get M, then divide each product by M. Enter the recipe on the mole basis here and read the mass column.

**How many gases can I enter?**
From 2 to 20 components, chosen from the 29-gas list.

**Can I get the mixture's density?**
At 0 °C and 1 atm, ideal density = M / 22.414 L/mol; air is 1.2923 kg/Nm³. For high-pressure mixtures a real-mixture model is needed.


## Sources

- [NIST Chemistry WebBook — Thermophysical Properties of Fluid Systems](https://webbook.nist.gov/chemistry/fluid/)
- [BIPM — The International System of Units (SI Brochure)](https://www.bipm.org/en/publications/si-brochure)

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