The mole fraction is a dimensionless quantity defined as which ratio?

• A. The ratio of mass to total mass
• B. The ratio of volume to total volume
• C. The ratio of moles to total moles
• D. The ratio of partial pressure to total pressure

Answer: (C)

Mole fraction expresses how much of a mixture, in terms of amount of substance, belongs to a particular component. It is defined as the moles of that component divided by the total moles of all components. Because both numerator and denominator are in moles, their units cancel, giving a pure number between 0 and 1—that is, a dimensionless quantity. This concept also ties directly to gas behavior: in an ideal mixture, the partial pressure of a component equals its mole fraction times the total pressure. It isn’t based on mass or volume, which would describe mass fraction or volume fraction instead. So the ratio of moles of the component to the total moles captures the correct definition of mole fraction.