Fração molar

Mole fraction (χ): composition of a solution

The mole fraction χᵢ of component i is just its moles divided by the total moles in the mixture: χᵢ = nᵢ / Σnⱼ. The value is dimensionless, sits somewhere between 0 and 1, and once you add up every component you get 1 (Σχᵢ = 1). Example: 1 mol of solute plus 9 mol of solvent gives χ_solute = 0.1 and χ_solvent = 0.9. Where molarity (mol/L) shifts with temperature and volume, the mole fraction doesn't budge, which is why it's the quantity you reach for in solution thermodynamics. Raoult's law: P = χ_solvent · P° (vapor pressure of an ideal solution).

Applications

It underpins solution thermodynamics (Raoult and Henry laws) and fractional distillation, where separating petroleum or ethanol by boiling point hangs on χ. It shows up in gas mixtures, from medical oxygen therapy O₂/N₂ to breathable atmospheres for diving, and in the chemical engineering of separation columns and reactors. For ideal gases, the mole fraction matches the volume fraction (Avogadro's law).

FAQ

Mole fraction vs molarity? Molarity rides on volume, and volume changes with T. Mole fraction is baked into the composition itself, so it works well for thermodynamic calculations and colligative properties.

Can χ exceed 1? No. By definition χ ∈ [0, 1]. A value of 1 means the pure substance, and a value of 0 means it isn't there at all.

Mole percent? Take χ · 100%. So χ = 0.1 comes out as 10 mol%.