Mixture Average Molar Mass Calculator

Enter the molar mass and composition of each component to compute the mixture’s average molar mass using mole fractions (or percentages). Works for gas mixtures, solution components, or any ideal mixture.

Background

The average molar mass of a mixture is the mole-fraction-weighted sum of the components’ molar masses: M̄ = Σ (x_i · M_i). You can enter mole fractions that sum to 1.000 or enter percentages that sum to 100%. The calculator will normalize and show each step clearly.

How to use this calculator

1) Add 2–6 components.
2) For each component, enter its molar mass (g/mol) and either a mole fraction (0–1) or a percent (0–100).
3) Click Calculate Average Molar Mass. The tool auto-detects whether you used fractions or percents and will normalize as needed.

Key Points

Mole fractions should sum to ≈1.000; percentages should sum to ≈100%.
If the total is slightly off (rounding), we’ll normalize it and tell you.
The final M̄ is reported in g/mol.

Formula & Equation Used

For a mixture with components i = 1…n:

\bar{M} = \sum_{i = 1}^{n} x_{i} \times M_{i}

x_i — mole fraction of component i (or %/100)
M_i — molar mass of component i (g/mol)

Example Problems

Example 1 (fractions):

Air model: N₂ (M = 28.014 g/mol, x = 0.78), O₂ (31.998, x = 0.21), Ar (39.948, x = 0.01)

M̄ = 0.78×28.014 + 0.21×31.998 + 0.01×39.948 = 28.96 g/mol (approx.)

Example 2 (percent):

Binary mixture: A (M = 50.0 g/mol, 40%), B (M = 100.0 g/mol, 60%)

M̄ = (0.40×50.0) + (0.60×100.0) = 20 + 60 = 80.0 g/mol

Frequently Asked Questions

Q: Can I enter percentages instead of mole fractions?

Yes. If your entries sum near 100, we’ll treat them as percentages and convert to fractions.

Q: What if my fractions don’t sum exactly to 1?

Small rounding errors are normalized automatically. We’ll show a note in the steps.

Q: What units are used for the result?

g/mol for the average molar mass.