Which term defines the fraction of moles that are solute relative to total moles in the solution?

• A. Mole fraction
• B. Molarity
• C. Molality
• D. Normality

Answer: (A)

The mole fraction is the fraction of moles that are solute relative to the total moles in the solution. It is defined as x_solute = n_solute / (n_solute + n_solvent + ...). For a simple binary solution, that becomes x_solute = n_solute / (n_solute + n_solvent). This quantity is dimensionless and always lies between 0 and 1, and the sum of the mole fractions of all components equals 1.

This measure lets you compare how much of each component exists on a molecular basis, independent of volume or mass. It’s especially useful in calculations involving partial pressures (Raoult’s law) and other colligative properties, where each component’s effect depends on its mole fraction in the mixture.

For example, if you have 1 mole of solute and 3 moles of solvent, the mole fraction of the solute is 1/(1+3) = 0.25. If you had 2 moles of solute and 3 moles of solvent, it would be 2/(2+3) = 0.40.

Molarity, molality, and normality describe concentrations in different units (per liter of solution, per kilogram of solvent, or per liter of solution in terms of equivalents), not the mole-based proportion within the mixture.