Ratio of the number of moles of substance in a solution to the total number of moles in the solution?

• A. Mole fraction
• B. Normality
• C. Boiling Point Elevation
• D. Mass percent

Answer: (A)

This is about mole fraction, which is the ratio of the amount of a component in moles to the total number of moles in the solution. It’s defined as x_i = n_i / n_total, where n_i is the moles of the component of interest and n_total is the sum of moles of all species in the mixture. This fraction tells you what portion of the mixture, in mole terms, is made up by that component and it always falls between 0 and 1, with all mole fractions summing to 1 across all components. Mole fraction is especially useful for predicting partial pressures and in applying Raoult’s law for ideal solutions. For example, if there are 2 moles of solute and 3 moles of solvent, the mole fraction of the solute is 2/5 = 0.4. Normality is a concentration measure in equivalents per liter, not a mole ratio to the total moles; boiling point elevation is a colligative property related to particle concentration, not a fraction of moles; and mass percent expresses proportions by mass, not by moles.