Which statement correctly contrasts molality and molarity?

Which statement correctly contrasts molality and molarity?

• A. Molality uses liters of solution; Molarity uses kilograms of solvent.
• B. Molality uses kilograms of solvent; Molarity uses liters of solution.
• C. Molality uses kilograms of solvent; Molarity uses grams per liter of solution.
• D. Molality uses moles of solvent per liter of solution.

Answer: (B)

The essential idea is how concentration is defined: molarity uses volume of the entire solution, while molality uses mass of the solvent. Molality is defined as moles of solute per kilogram of solvent, whereas molarity is moles of solute per liter of solution. This distinction matters because volume can change with temperature, so molarity is temperature-dependent, while mass of solvent stays essentially constant, making molality temperature-independent. That’s why the correct contrast is that molality uses kilograms of solvent and molarity uses liters of solution. The other statements mix up these definitions: using liters of solution for molality or kilograms of solvent for molarity would be incorrect, and describing molarity in terms of grams per liter mislabels it as a mass-per-volume measure rather than a mole-per-volume measure.

The essential idea is how concentration is defined: molarity uses volume of the entire solution, while molality uses mass of the solvent. Molality is defined as moles of solute per kilogram of solvent, whereas molarity is moles of solute per liter of solution. This distinction matters because volume can change with temperature, so molarity is temperature-dependent, while mass of solvent stays essentially constant, making molality temperature-independent.

That’s why the correct contrast is that molality uses kilograms of solvent and molarity uses liters of solution. The other statements mix up these definitions: using liters of solution for molality or kilograms of solvent for molarity would be incorrect, and describing molarity in terms of grams per liter mislabels it as a mass-per-volume measure rather than a mole-per-volume measure.