mole fraction
/ mohl FRAK-shun /
Suppose you could line up every particle in a mixture and ask: out of all of them, what share belongs to this one component? That share, counted in particles rather than weight or volume, is the mole fraction.
Mole fraction is the moles of one component divided by the total moles of everything present. It is a pure number with no units, always between zero and one, and the mole fractions of all components in a mixture must add up to exactly one.
Because it is built purely on counting particles, mole fraction is unaffected by temperature and pressure, which makes it the natural unit for gas mixtures and for theory like Raoult's law of vapour pressure. The trade-off is that you must know the composition in moles, which means weighing and converting through molar masses first.
A mixture of 2 moles of ethanol and 8 moles of water has 10 moles in total, so ethanol's mole fraction is 2 ÷ 10 = 0.2 and water's is 0.8 — together they sum to 1.
All mole fractions in a mixture add up to exactly one.
Mole fraction counts particles, while weight percent weighs them. For a mixture of light and heavy molecules, the two can differ a lot, so never substitute one for the other.