Which statement distinguishes molarity from molality?

Which statement distinguishes molarity from molality?

• A. Molarity uses volume of solution; Molality uses mass of solvent
• B. Molality uses volume; Molarity uses mass
• C. Both depend on temperature the same way
• D. They are identical

Answer: (A)

The key idea is what each concentration uses as the reference amount: Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent. That means the statement in question is describing the fundamental distinction: molarity depends on the volume of the solution, whereas molality depends on the mass of the solvent. Temperature plays a big role here. Volume changes with temperature because liquids expand or contract, so the amount of solute per liter of solution (the molarity) can change as temperature changes. Mass, on the other hand, stays essentially the same as temperature varies (ignoring tiny changes due to phase changes), so the amount of solute per kilogram of solvent (the molality) stays effectively constant. This makes molality more reliable for comparing concentrations across different temperatures, which is why it's often used in colligative property calculations. So the correct statement captures that molarity uses volume of the solution, while molality uses mass of the solvent. The other ideas—swapping volume and mass, claiming they depend on temperature in the same way, or saying they’re identical—don’t fit because they misstate what each unit measures or overlook the temperature sensitivity of volume.

The key idea is what each concentration uses as the reference amount: Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent. That means the statement in question is describing the fundamental distinction: molarity depends on the volume of the solution, whereas molality depends on the mass of the solvent.

Temperature plays a big role here. Volume changes with temperature because liquids expand or contract, so the amount of solute per liter of solution (the molarity) can change as temperature changes. Mass, on the other hand, stays essentially the same as temperature varies (ignoring tiny changes due to phase changes), so the amount of solute per kilogram of solvent (the molality) stays effectively constant. This makes molality more reliable for comparing concentrations across different temperatures, which is why it's often used in colligative property calculations.

So the correct statement captures that molarity uses volume of the solution, while molality uses mass of the solvent. The other ideas—swapping volume and mass, claiming they depend on temperature in the same way, or saying they’re identical—don’t fit because they misstate what each unit measures or overlook the temperature sensitivity of volume.