Mass Fraction to Mole Fraction Calculator

Example Data Table

This example uses three common liquids with mass fractions that sum to 1.

Formula Used

For a mixture with mass fractions wᵢ and molar masses Mᵢ, first compute moles proportional to each component:

• wᵢ = mass fraction of component i (fraction or percent).
• Mᵢ = molar mass of component i.
• m_total = total mixture mass (used for moles).
• xᵢ = mole fraction of component i.

How to Use This Calculator

1. Select whether your mass fractions are fractions or percent.
2. Enter the total mixture mass and its unit.
3. Add components and enter mass fraction and molar mass.
4. Enable normalization if your fractions may not sum exactly.
5. Click Calculate to see mole fractions and mole percent.
6. Use Download CSV or Download PDF for reporting.

Why mole fraction matters in mixtures

Many property models depend on mole fraction, not mass fraction. Raoult’s law, Dalton’s law, and reaction stoichiometry use moles as the counting unit. A component that is heavy can dominate mass but still contribute fewer molecules. For example, 50% acetone by mass contains far fewer molecules than 50% water by mass. Converting composition helps you compare mixtures on a molecular basis.

Core relationship behind the conversion

Start with a mass fraction wᵢ and molar mass Mᵢ. The moles are proportional to wᵢ/Mᵢ, so the total mass cancels when you compute xᵢ. This means you can obtain mole fractions even if you only know relative mass fractions, provided molar masses are correct. If you use percent inputs, convert them to fractions first by dividing by 100.

Picking a practical mass basis

Although xᵢ does not require a specific mass, the calculator also reports masses and moles. A convenient basis is 100 g or 1 kg, because wᵢ × basis gives component mass directly. If you measured 250 g of mixture, enter 250 g to get real moles for material balances and dosing.

Normalization and tolerance

Measured fractions often sum to 0.997 or 1.012 due to rounding. With normalization enabled, each wᵢ is scaled by the total Σw to force Σw = 1. The page also warns you when the mismatch exceeds a small tolerance (about 0.5%). If you disable normalization, you must correct the input totals yourself.

Fraction vs percent entry

Use “Fraction” when inputs sum to 1.000, like 0.25, 0.50, 0.25. Use “Percent” when inputs sum to 100, like 25, 50, 25. The calculator converts percent to fraction internally, so the math stays consistent across both modes, and the final mole percent still sums to 100%.

Molar mass units and useful reference values

Enter molar mass as g/mol or kg/kmol; both share the same numeric value. For example, water is 18.015, ethanol is 46.07, and acetone is 58.08. Air is often approximated as 28.97, and carbon dioxide is 44.01. If a molar mass is wrong, xᵢ shifts strongly, especially for light gases and low‑MW solvents.

Reading the results table

The output includes normalized wᵢ, mass used, moles, xᵢ, and mole percent. Mole percent is simply 100 × xᵢ and should add to 100%. If one component has a very small xᵢ, consider using more significant digits in your inputs and verifying units to reduce rounding drift.

Where this conversion is commonly used

Chemical engineering uses mole fraction for phase equilibrium, mixture molecular weight, and diffusion calculations. Lab work uses it for preparing solutions from mass-based recipes, then comparing with literature data reported in mole fraction. The built‑in checks help catch negative fractions, missing components, and non‑positive molar masses before you export results for a report.

1) Can I convert mass percent directly to mole fraction?

Yes. Choose the percent basis, enter values that sum to 100, and provide molar masses. The calculator converts percent to fractions internally and then computes mole fractions and mole percent.

2) Do I need to enter total mixture mass to get mole fractions?

No for xᵢ, because xᵢ depends on ratios (wᵢ/Mᵢ). However, total mass is needed to report component masses and absolute moles, which are useful for material balances and reporting.

3) What happens if my mass fractions do not sum exactly to 1 or 100?

If normalization is enabled, the tool rescales all fractions so their sum becomes exactly 1. If normalization is disabled, the tool will stop and ask you to correct the totals.

4) Are g/mol and kg/kmol different for this calculation?

Numerically they are identical, because 1 kg/kmol equals 1 g/mol. Pick the label you prefer, but keep your component values consistent and positive.

5) Why does a heavier component often have a lower mole fraction?

Mole fraction counts molecules. For the same mass share, a high molar mass means fewer moles. That lowers its mole fraction compared with a lighter component.

6) How many components can I include?

You can add as many rows as you need. For best readability, keep names short and use realistic molar masses. The CSV and PDF exports include every component you enter.

7) How can I verify the results quickly?

Check that mole percent sums to about 100%, and that components with smaller molar mass generally gain mole fraction compared with their mass fraction. You can also compare against a hand calculation using xᵢ ∝ wᵢ/Mᵢ.