mole fraction

Meaning: The mole fraction of a component is its share of all the particles in a mixture. Because it is a ratio of moles, it has no units, and it does not change with temperature or volume. The mole fractions of every component add up to 1. In a gas mixture it fixes each gas's partial pressure. In an ideal solution it fixes each volatile component's vapor pressure.

Equation: XA = nA / ntotal, where XA is the mole fraction of A (unitless), nA is the moles of A (mol), and ntotal is the sum of the moles of all components (mol). Two common uses are PA = XA × Ptotal (Dalton's law) and PA = XA × P°A (Raoult's law), where PA is the partial or vapor pressure of A, Ptotal is the total pressure, and P°A is the vapor pressure of pure A, all in Pa.

On the MCAT: A passage gives masses, volumes or concentrations. Convert to moles first, then divide. You may be asked to find a gas's partial pressure from its mole fraction, or to predict that adding a nonvolatile solute lowers the solvent's mole fraction and therefore its vapor pressure.

Common confusion: Mole fraction is not molarity, molality or mass fraction. Equal masses of two substances rarely give equal mole fractions, because molar masses differ, so always convert to moles. A solute that dissociates, such as NaCl, counts each ion separately in the total moles.