Mole fraction is a concentration unit that is defined as:

In these formulas, x represents the mole fraction, and the various n's represent the total number of moles of each component.

Mole fraction is very useful when understanding selected solution properties such as vapor pressure.

This unit of concentration precisely describes the amount of both solute and solvent in contrast to molarity where the exact amount of solvent is difficult to ascertain in a sample of the solution.

When substances are ionic, the number of moles of ions in aqueous solutions are treated separately because the ions are discrete species in water solutions. Each ion changes the properties of the solution. When one mole of KBr dissolves in water, 2 moles of solute are formed - one mole of potassium ion and one mole of bromide. The total moles in the solution is the number of moles of water plus the moles of potassium ion plus the moles of bromide.