Which quantity expresses concentration as a mass percentage?

• A. Molarity
• B. Mass percent
• C. Molality
• D. Mole fraction

Answer: (B)

Mass percent expresses concentration as a mass percentage. It compares the mass of the solute to the total mass of the solution and multiplies by 100, so mass percent = (mass of solute / mass of solution) × 100, with the solution’s mass being solute plus solvent. This directly shows what fraction of the solution’s mass comes from the dissolved substance. For example, dissolving 5 g of solute in 95 g of solvent gives a total mass of 100 g, so the mass percent is 5%. Other concentration measures use different bases: molarity is moles of solute per liter of solution (depends on volume), molality is moles of solute per kilogram of solvent (depends on solvent mass but is not a percent), and mole fraction is the ratio of moles of solute to total moles (a dimensionless fraction).