Which statement correctly defines molality and molarity?

• A. Molality equals moles solute per liter of solution; Molarity equals moles solute per kilogram of solvent.
• B. Molality equals moles solute per kilogram of solvent; Molarity equals moles solute per liter of solution.
• C. Molality equals mass percent; Molarity equals mole fraction.
• D. Molality equals moles solute per gram of solvent; Molarity equals moles solute per liter of solvent.

Answer: (B)

Two common ways to express concentration are molality and molarity. Molality uses moles of solute per kilogram of solvent, which makes it independent of volume changes. Molarity uses moles of solute per liter of solution, so the amount per unit volume of the solution is what you’re measuring, and that can vary with temperature.

The statement that matches this is: moles of solute per kilogram of solvent for molality, and moles of solute per liter of solution for molarity. The other options mix up the units or refer to different concentration measures, such as mass percent or mole fraction, which are not definitions of these two quantities.