Mole Fraction Calculator
The mole fraction of each component in a two-part mixture — from masses or moles, with the mass fraction alongside.
Mole fraction is a component's moles divided by the total moles: χ_A = n_A/(n_A + n_B).
How the mole fraction calculator works
Mole fraction is a component's moles divided by the total moles: χ_A = n_A/(n_A + n_B). Being a ratio of counts it is dimensionless, and the fractions always sum to 1.
It differs sharply from mass fraction when the molar masses differ. Equal masses of water and ethanol are 50% each by mass but 71% water by moles, because water molecules are so much lighter.
Formula: χ_A = n_A / (n_A + n_B)
Worked examples
| Inputs | Mole fraction of A | Note |
|---|---|---|
| Equal masses of water and ethanol | 0.718889 | 71% water by moles, 50% by mass |
| Two moles to one | 0.666667 | χA = 0.667 |
| Pure A | 1 | χA = 1 |
FAQFrequently asked questions
What is mole fraction?
A component's share of the total moles in a mixture. The fractions always sum to 1.
How is it different from mass fraction?
It counts molecules rather than weighing them. When molar masses differ the two diverge sharply.
Where is it used?
Raoult's law, partial pressures, phase diagrams — anywhere the number of particles matters more than their weight.
Can it exceed 1?
No. It is a share of a total, so it is always between 0 and 1.
What is mole percent?
The same thing times 100. Gas compositions are usually quoted this way — air is 78 mole percent nitrogen.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.