Mole Fraction

x_1 = \frac{n_1}{n_1 + n_2}

Worked example: 1 mol solute + 9 mol solvent → x1 = 0.1 — press Try an example to run it live, then adjust anything.

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Mole Fraction explained

The mole fraction asks the simplest possible concentration question: of every mole of particles in the mixture, what share belongs to component 1? Dissolve 1.00 mol of ethylene glycol in 9.00 mol of water and x₁ = 1.00/(1.00 + 9.00) = 0.100 — no units, no temperature dependence, and all the mole fractions in a mixture always sum to exactly 1.

This makes x the natural currency of vapor-pressure laws: Raoult's law and Dalton's law are both written in mole fractions, and gas mixtures are routinely quoted this way — dry air is x ≈ 0.78 nitrogen and 0.21 oxygen. Solving the definition backwards for n₁ or n₂ requires x strictly between 0 and 1, since a pure component (x = 1) carries no information about how much solvent could be present.

Mole Fraction formula

x_1 = \frac{n_1}{n_1 + n_2}

Where

$x_1$ = Mole fraction of solute
$n_1$ = Amount of solute (mol)
$n_2$ = Amount of solvent (mol)

Missing one of these? Work it out first, then come back

Mole fraction of solute — Raoult's Law, Partial Pressure from Mole Fraction
Amount of solute — Molarity (C = n/V), Molality (b = n/m)
Amount of solvent — Ideal Gas Law, Moles from Mass (n = m/M)

Part of this formula set

Ways to express concentration

6 formulasMolarity, molality, mass percent, mole fraction, normality and the dilution equation — six answers to “how much solute is in there?”